patch 8.2.0608: warning from clang when building message test

Problem:    Warning from clang when building message test.
Solution:   Use a void pointer.  (Dominique Pelle, closes #5958)

src/message_test.c

@@ -127,7 +127,7 @@ test_vim_snprintf(void)
     int		n;
     size_t	bsize;
     int		bsize_int;
-    char	*ptr = (char *)0x87654321;
+    void	*ptr = (void *)0x87654321;
 
     // Loop on various buffer sizes to make sure that truncation of
     // vim_snprintf() is correct.