Avoid minor leak in parallel pg_dump

During parallel pg_dump, a worker process closing the connection caused
a minor memory leak (particularly minor as we are likely about to exit
anyway).  Instead, free the memory in this case prior to returning NULL
to indicate connection closed.

Spotting by the Coverity scanner.

Back patch to 9.3 where this was introduced.

src/bin/pg_dump/parallel.c

@@ -1288,7 +1288,7 @@ readMessageFromPipe(int fd)
 
 		/* worker has closed the connection or another error happened */
 		if (ret <= 0)
-			return NULL;
+			break;
 
 		Assert(ret == 1);
 
@@ -1303,6 +1303,14 @@ readMessageFromPipe(int fd)
 			msg = (char *) realloc(msg, bufsize);
 		}
 	}
+
+	/*
+	 * Worker has closed the connection, make sure to clean up before return
+	 * since we are not returning msg (but did allocate it).
+	 */
+	free(msg);
+
+	return NULL;
 }
 
 #ifdef WIN32