Ignore GCC memory overflow warnings when testing overflows

We know we are overflowing the maximum allocation: it's what we're
testing for.

https://bugzilla.gnome.org/show_bug.cgi?id=794732

glib/tests/mem-overflow.c

@@ -23,6 +23,8 @@
 
 #ifdef __GNUC__
 #pragma GCC optimize (1)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Walloc-size-larger-than="
 #endif
 
 #include "glib.h"
@@ -198,6 +200,10 @@ empty_alloc (void)
 }
 #endif
 
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
+
 int
 main (int   argc,
       char *argv[])