Fix a null-deref-on-fail in unit tests

If geoip_format_bridge_stats() returned NULL when it should have
returned a string, we would have tried to deref NULL, and died.  Not
a big deal in the unit tests, but still worth fixing.

Found by coverity; This is CID 743384.

src/test/test.c

@@ -1678,6 +1678,7 @@ test_geoip(void)
    * the connecting clients added above. */
   geoip_bridge_stats_init(now);
   s = geoip_format_bridge_stats(now + 86400);
+  test_assert(s);
   test_streq(bridge_stats_1, s);
   tor_free(s);