During parallel pg_dump, free commands from master

The command strings read by the child processes during parallel
pg_dump, after being read and handled, were not being free'd.
This patch corrects this relatively minor memory leak.

Leak found by the Coverity scanner.

Back patch to 9.3 where parallel pg_dump was introduced.

Author: sfrost

src/bin/pg_dump/parallel.c

@@ -922,6 +922,9 @@ WaitForCommands(ArchiveHandle *AH, int pipefd[2])
 			exit_horribly(modulename,
 						  "unrecognized command on communication channel: %s\n",
 						  command);
+
+		/* command was pg_malloc'd and we are responsible for free()ing it. */
+		free(command);
 	}
 }