Don't crash on xrealloc(0) (found by Jon Griffiths).

tools/winebuild/utils.c

diff --git a/tools/winebuild/utils.c b/tools/winebuild/utils.c
index f63c85c..0bdd0a3 100644
--- a/tools/winebuild/utils.c
+++ b/tools/winebuild/utils.c
@@ -45,7 +45,7 @@
 void *xrealloc (void *ptr, size_t size)
 {
     void *res = realloc (ptr, size);
-    if (res == NULL)
+    if (size && res == NULL)
     {
         fprintf (stderr, "Virtual memory exhausted.\n");
         exit (1);