renderer: fix potential leak

Found thanks to llvm scan-build.

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

src/vrend_renderer.c

@@ -5039,6 +5039,7 @@ void vrend_set_streamout_targets(struct vrend_context *ctx,
          target = vrend_object_lookup(ctx->sub->object_hash, handles[i], VIRGL_OBJECT_STREAMOUT_TARGET);
          if (!target) {
             report_context_error(ctx, VIRGL_ERROR_CTX_ILLEGAL_HANDLE, handles[i]);
+            free(obj);
             return;
          }
          vrend_so_target_reference(&obj->so_targets[i], target);