vbo: init prim[] array with memset()

This fixes a Coverity warning about uninitialized data.

src/mesa/vbo/vbo_exec_array.c

@@ -585,15 +585,11 @@ vbo_draw_arrays(struct gl_context *ctx, GLenum mode, GLint start,
    if (ctx->NewState)
       _mesa_update_state(ctx);
 
+   /* init most fields to zero */
+   memset(prim, 0, sizeof(prim));
    prim[0].begin = 1;
    prim[0].end = 1;
-   prim[0].weak = 0;
-   prim[0].pad = 0;
    prim[0].mode = mode;
-   prim[0].start = 0; /* filled in below */
-   prim[0].count = 0; /* filled in below */
-   prim[0].indexed = 0;
-   prim[0].basevertex = 0;
    prim[0].num_instances = numInstances;
 
    /* Implement the primitive restart index */