mesa/util: don't ignore NULL returned from 'malloc'

We should exit from the function 'util_vasprintf'
with error code -1 for case where 'malloc'
returns NULL

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Fixes: 864148d69e "util: add util_vasprintf() for Windows (v2)"
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
(cherry picked from commit 65cfe698b0)

src/util/u_string.h

@@ -126,7 +126,7 @@ util_vasprintf(char **ret, const char *format, va_list ap)
       return -1;
 
    *ret = (char *) malloc(r + 1);
-   if (!ret)
+   if (!*ret)
       return -1;
 
    /* Print to buffer */