vtest: be reasonable about cmd length

Avoid overflow, found thanks to AddressSanitizer & american fuzzy lop.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

vtest/vtest_renderer.c

@@ -26,6 +26,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <limits.h>
 
 #include "virglrenderer.h"
 
@@ -228,6 +229,9 @@ int vtest_submit_cmd(uint32_t length_dw)
     uint32_t *cbuf;
     int ret;
 
+    if (length_dw > UINT_MAX / 4)
+       return -1;
+
     cbuf = malloc(length_dw * 4);
     if (!cbuf)
 	return -1;