elasticarray: clarify unsigned arithmetic wrap around is handled

This commit adds a code comment describing the cases handled by the
if (nalloc < nsize) block. One such case is the potential wrap around
of EA->alloc * 2. Without the code comment, it was not quite apparent
upon seeing the code that the wrap around is properly handled.

libcperciva/datastruct/elasticarray.c

@@ -28,6 +28,12 @@ resize(struct elasticarray * EA, size_t nsize)
 	if (EA->alloc < nsize) {
 		/* We need to enlarge the buffer. */
 		nalloc = EA->alloc * 2;
+		/*
+		 * Handle if nalloc is not large enough to hold nsize which can
+		 * happen when: 1) EA->alloc is small enough that EA->alloc * 2
+		 * is still smaller than nsize, or 2) EA->alloc is large enough
+		 * that EA->alloc * 2 wraps around becoming smaller than nsize.
+		 */
 		if (nalloc < nsize)
 			nalloc = nsize;
 	} else if (EA->alloc / 4 > nsize) {