The story (at least this paragraph of it) is in the past. Verb tense is generally consistent:
watched, looked, cursed, tainted, stopped, would (not) last, was, was picked, was
There is one clear exception to this consistency. The verb of matrix clause of the sentence in question is "know" -- a present tense verb. This works because the narrator is offering his present tense insight regarding the past tense situation.
if my lord had the food there
In context, this is a past tense clause. One clear present tense version would be "if my lord has the food there".
Your proposed substitution is sensible, but it is not strictly necessary. The clause "if my lord had had the food there" is also a past tense clause and would also be consistent with the tense of the overall passage. However, this substitution does change something. It changes the aspect. The "had" is a past tense and indefinite aspect construction, just like the other past tense verbs listed above. The "had had" is a past tense and perfect aspect construction.
There is one perfect construction in the original passage: "he would have shared it gladly". Taken on its own, this clause does not have a clear tense. The auxiliary "would" could be a past tense form, a subjunctive mode form, or both. In context, it seems to be both. The past tense makes sense given the tense of the overall passage, and the subjunctive mode makes sense given that this clause is the result of a condition that was contrary to fact. He didn't have the food, so he couldn't share it.
We could, as an exercise, cast the entire paragraph (except for the narrator's simple insight) in the perfect aspect.
I had watched sadly as my lord Garridan had looked out at the withered fields before him from his meager stone keep and had cursed the luck that had tainted the skies and had stopped the rain from falling. The families in his charge would not have lasted the winter, which had always been bitter and cold in the northern reaches of the Jerals. His own supply of grain had already been picked clean; there had been barely enough to sustain him for the months ahead. I know if my lord had had the food there, he would have shared it gladly, allowing his charges to pay him back in whatever time or manner they could have afforded....
A similar exercise involves casting the paragraph in the present tense:
I watch sadly as my lord Garridan looks out at the withered fields before him from his meager stone keep and curses the luck that taints the skies and stops the rain from falling. The families in his charge will not last the winter, which is always bitter and cold in the northern reaches of the Jerals. His own supply of grain is already picked clean; there is barely enough to sustain him for the months ahead. I know if my lord had the food there, he would share it gladly, allowing his charges to pay him back in whatever time or manner they can afford . . . and in some cases, to those in dire need, give it to them without costs. Something has to be done; and it has to be done soon.
In this case, "if my lord had the food there" is a present tense construction. The form "had" shares a certain ambiguity with the form "would". For these verbs (as is true of most verbs in English) the subjunctive form is identical to the past tense form. We cannot determine the tense or mode of those forms without context.
Your proposed substitution avoids that ambiguity. The clause "if my lord had had the food there" definitely employs both the past tense and the perfect aspect. There is no possibility of that construction being a present tense subjunctive.
A present tense subjunctive simply doesn't make sense for the conditional clause in the original passage. The past tense of the context makes the present tense interpretation of "if he had" impossible. The technical ambiguity is not a practical ambiguity.
Is it better style to avoid the technical ambiguity, or to avoid the unnecessary perfect aspect? That's a reasonable question, but it has no universal answer. What counts as good style changes from dialect to dialect and even from person to person.
