Fix buffer underrun noted and solved by Larry Doolittle.
diff --git a/dirname.c b/dirname.c
index 8313e99..935a831 100644
--- a/dirname.c
+++ b/dirname.c
@@ -37,7 +37,7 @@
 	argv++;
 
 	s=*argv+strlen(*argv)-1;
-	while (s != *argv && *s == '/') {
+	while (s > *argv && *s == '/') {
 		*s-- = '\0';
 	}
 	s = strrchr(*argv, '/');