lockgetpc may not panic if the given lock is nil.  this is because
lockgetpc() is called from procdump, where it is not know if
the lock is nil or not.

for good measure and symmetry, locksetpc will ignore a nil lock.

Reference: /n/atom/patch/applied/mcslockttp
Date: Thu May 15 04:37:21 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/port/mcslock.c	Thu May 15 04:35:27 2014
+++ /sys/src/nix/port/mcslock.c	Thu May 15 04:35:28 2014
@@ -203,8 +203,7 @@
 {
 	LockEntry *ql;
 
-	ql = l->e;
-	if(ql != nil && ql->used == l)
+	if(l != nil && (ql = l->e) != nil && ql->used == l)
 		return ql->pc;
 	return 0;
 }
@@ -214,7 +213,6 @@
 {
 	LockEntry *ql;
 
-	ql = l->e;
-	if(ql != nil && ql->used == l && ql->m == m)
+	if(l != nil && (ql = l->e) != nil  && ql->used == l && ql->m == m)
 		ql->pc = pc;
 }