minor tweaks

R=r
DELTA=9  (2 added, 5 deleted, 2 changed)
OCL=24107
CL=24152
diff --git a/src/runtime/string.c b/src/runtime/string.c
index e708d02..001050b 100644
--- a/src/runtime/string.c
+++ b/src/runtime/string.c
@@ -12,6 +12,8 @@
 {
 	int32 l;
 
+	if(s == nil)
+		return 0;
 	for(l=0; s[l]!=0; l++)
 		;
 	return l;