BUG/MINOR: mworker: fix a memleak when execvp() failed

Free next_argv when execvp() failed.

Must be backported as far as 1.8.

Should fix issue #668.
diff --git a/src/haproxy.c b/src/haproxy.c
index 92a54ba..5d7b0fe 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -844,6 +844,8 @@
 	execvp(next_argv[0], next_argv);
 
 	ha_warning("Failed to reexecute the master process [%d]: %s\n", pid, strerror(errno));
+	free(next_argv);
+	next_argv = NULL;
 	return;
 
 alloc_error: