convert trivial cases to ALLOC_ARRAY

Each of these cases can be converted to use ALLOC_ARRAY or
REALLOC_ARRAY, which has two advantages:

  1. It automatically checks the array-size multiplication
     for overflow.

  2. It always uses sizeof(*array) for the element-size,
     so that it can never go out of sync with the declared
     type of the array.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/sha1_file.c b/sha1_file.c
index 73ccd49..d14abfe 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -1942,7 +1942,7 @@
 		/* Push the object we're going to leave behind */
 		if (poi_stack_nr >= poi_stack_alloc && poi_stack == small_poi_stack) {
 			poi_stack_alloc = alloc_nr(poi_stack_nr);
-			poi_stack = xmalloc(sizeof(off_t)*poi_stack_alloc);
+			ALLOC_ARRAY(poi_stack, poi_stack_alloc);
 			memcpy(poi_stack, small_poi_stack, sizeof(off_t)*poi_stack_nr);
 		} else {
 			ALLOC_GROW(poi_stack, poi_stack_nr+1, poi_stack_alloc);
@@ -2308,7 +2308,7 @@
 		if (delta_stack_nr >= delta_stack_alloc
 		    && delta_stack == small_delta_stack) {
 			delta_stack_alloc = alloc_nr(delta_stack_nr);
-			delta_stack = xmalloc(sizeof(*delta_stack)*delta_stack_alloc);
+			ALLOC_ARRAY(delta_stack, delta_stack_alloc);
 			memcpy(delta_stack, small_delta_stack,
 			       sizeof(*delta_stack)*delta_stack_nr);
 		} else {