Fix potential out-of-bounds access in strncmp()

The current strncmp() implementation may continue comparing strings
when they are identical and shorter than 'len', leading to potential
out-of-bounds memory access. For example, strncmp("abc", "abc", 5)
could access memory beyond the end of the strings.

Add a check to return 0 if the end of either string is reached before
'len', preventing unintended memory access.

lib/c.c

@@ -71,6 +71,8 @@ int strncmp(char *s1, char *s2, int len)
             return -1;
         if (s1[i] > s2[i])
             return 1;
+        if (!s1[i])
+            return 0;
         i++;
     }
     return 0;