svga: Prevent use after free.

Courtesy of clang static analyzer.

I was hunting for potential sources of memory corruption using Mesa with
a GL trace, and happened to find this (unrelated) issue.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

src/gallium/drivers/svga/svga_context.c

@@ -327,6 +327,7 @@ cleanup:
    util_bitmask_destroy(svga->stream_output_id_bm);
    util_bitmask_destroy(svga->query_id_bm);
    FREE(svga);
+   svga = NULL;
 
 done:
    SVGA_STATS_TIME_POP(svgascreen->sws);