Fix a bug which references undefined variable `ptr` (#195)

diff --git a/rpmalloc/rpmalloc.c b/rpmalloc/rpmalloc.c
index 975b341..966a80e 100644
--- a/rpmalloc/rpmalloc.c
+++ b/rpmalloc/rpmalloc.c
@@ -3100,7 +3100,7 @@
 #if ENABLE_VALIDATE_ARGS
 	if (size >= MAX_ALLOC_SIZE) {
 		errno = EINVAL;
-		return ptr;
+		return 0;
 	}
 #endif
 	return _rpmalloc_allocate(heap, size);
@@ -3111,7 +3111,7 @@
 #if ENABLE_VALIDATE_ARGS
 	if (size >= MAX_ALLOC_SIZE) {
 		errno = EINVAL;
-		return ptr;
+		return 0;
 	}
 #endif
 	return _rpmalloc_aligned_allocate(heap, alignment, size);