psql: Fix memory leak with \gx used within a pipeline

While inside a pipeline, \gx is currently forbidden and will make
exec_command_g() exit early.  There was a memory leak in this code path,
so let's fix it.

Author: Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>
Discussion: https://postgr.es/m/CAO6_XqqFVQjLjZQiL7xdwLpzZEy1ghO_JWvCFPM_OmwF9s7XdA@mail.gmail.com

Author: michaelpq

src/bin/psql/command.c

@@ -1739,6 +1739,7 @@ exec_command_g(PsqlScanState scan_state, bool active_branch, const char *cmd)
 		{
 			pg_log_error("\\gx not allowed in pipeline mode");
 			clean_extended_state();
+			free(fname);
 			return PSQL_CMD_ERROR;
 		}