fix crash when the liblinphone_tester is executed several time in the same process

mediastreamer2 @ 95cbe2f5

-Subproject commit 304b07fbe4fb681cf32d32cb124076d00ffde335
+Subproject commit 95cbe2f55a5802d3260a44b8ec28443a2e2e7940

tester/tester.c

@@ -533,6 +533,7 @@ void liblinphone_tester_uninit(void) {
 	if (all_leaks_buffer) {
 		bc_tester_printf(bc_printf_verbosity_info, all_leaks_buffer);
 		ms_free(all_leaks_buffer);
+		all_leaks_buffer = NULL;
 	}
 	bc_tester_uninit();
 }