avoid zombie when execvp fails

src/libthread/exec.c

diff --git a/src/libthread/exec.c b/src/libthread/exec.c
index d745248..a6d536e 100644
--- a/src/libthread/exec.c
+++ b/src/libthread/exec.c
@@ -93,6 +93,7 @@
 	n = read(p[0], exitstr, sizeof exitstr-1);
 	close(p[0]);
 	if(n > 0){	/* exec failed */
+		free(waitfor(pid));
 		exitstr[n] = 0;
 		errno = atoi(exitstr);
 		return -1;