string: fix strtok

After a call to `strtok`, the internal state is left to \0; this leads
to returning NULL on the next call regardless of the string.

Signed-off-by: Daniele Ahmed <ahmeddan amazon c;0m >

include/string.h

@@ -292,7 +292,7 @@ static inline char *strtok(char *s, const char *delim) {
     int ch;
 
     if (NULL == s)
-        s = lasts;
+        s = lasts + 1;
 
     do {
         if ((ch = *s++) == '\0')