vrend: check if we got a version string

Otherwise the code will simply crash, we should give it a chance to
exit cleanly.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Rohan Garg <rohan.garg@collabora.com>

src/vrend_renderer.c

@@ -6064,6 +6064,8 @@ static bool use_integer() {
       return true;
 
    const char * a = (const char *) glGetString(GL_VENDOR);
+   if (!a)
+       return false;
    if (strcmp(a, "ARM") == 0)
       return true;
    return false;