dftest: Always free the get_args_as_string result

Fixes ASAN test failures for 14 dfilter tests.

Change-Id: I1592bb1f679253a5eb8be8496881c4e4af207612
Reviewed-on: https://code.wireshark.org/review/31843
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Peter Wu <peter@lekensteyn.nl>

dftest.c

@@ -107,6 +107,7 @@ main(int argc, char **argv)
 		fprintf(stderr, "dftest: %s\n", err_msg);
 		g_free(err_msg);
 		epan_cleanup();
+		g_free(text);
 		exit(2);
 	}
 
@@ -119,6 +120,7 @@ main(int argc, char **argv)
 
 	dfilter_free(df);
 	epan_cleanup();
+	g_free(text);
 	exit(0);
 }