shallow.c: avoid theoretical pointer wrap-around

The expression info->free+size is technically undefined behaviour in
exactly the case we want to test for. Moreover, the compiler is likely
to translate the expression to

  (unsigned long)info->free + size > (unsigned long)info->end

where there's at least a theoretical chance that the LHS could wrap
around 0, giving a false negative.

This might as well be written using pointer subtraction avoiding these
issues.

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

shallow.c

@@ -368,7 +368,7 @@ static uint32_t *paint_alloc(struct paint_info *info)
 	unsigned nr = (info->nr_bits + 31) / 32;
 	unsigned size = nr * sizeof(uint32_t);
 	void *p;
-	if (!info->pool_count || info->free + size > info->end) {
+	if (!info->pool_count || size > info->end - info->free) {
 		if (size > POOL_SIZE)
 			die("BUG: pool size too small for %d in paint_alloc()",
 			    size);