Fix strerror(3) for errno 0.

Everyone else's C library says "Success". We say "Unknown error: 0", which
isn't really true.

Change-Id: I9f9054779123eda996634e5f7a277789b6805809

libc/string/strerror_r.c

@@ -21,7 +21,7 @@ __code_string_lookup( const CodeString*  strings,
 
     for (;;)
     {
-        if (strings[nn].code == 0)
+        if (strings[nn].msg == NULL)
             break;
 
         if (strings[nn].code == code)