vrend_renderer: check for 0 length shader request.

This is a follow up change for a5bfadaab5
to fix some request with 0 length shader. This moves the check from the
decode phase into create_shader as userspace may send 0
length shader in a long shader request.

TEST=webglaquarium
TEST=piglit opengl -t glsl --glsl

Reviewed-by: Dave Airlie <airlied@redhat.com>

src/vrend_renderer.c

@@ -3053,8 +3053,8 @@ int vrend_create_shader(struct vrend_context *ctx,
       struct tgsi_token *tokens;
 
       /* check for null termination */
-      int last_chunk_offset = sel->buf_offset ? sel->buf_offset : pkt_length * 4;
-      if (!memchr(shd_text + last_chunk_offset - 4, '\0', 4)) {
+      uint32_t last_chunk_offset = sel->buf_offset ? sel->buf_offset : pkt_length * 4;
+      if (last_chunk_offset < 4 || !memchr(shd_text + last_chunk_offset - 4, '\0', 4)) {
          ret = EINVAL;
          goto error;
       }