winetest: Avoid an infinite loop in vstrfmtmake() when the format is NULL.

When the format is NULL vsnprintf() always returns -1 so we keep
doubling the allocated memory until running out of memory.

programs/winetest/util.c

@@ -56,6 +56,7 @@ static char *vstrfmtmake (size_t *lenp, const char *fmt, va_list ap)
     char *p;
     int n;
 
+    if (!fmt) fmt = "";
     p = xalloc(size);
     while (1) {
         n = vsnprintf (p, size, fmt, ap);