When decoding, prefer ptrdiff_t to int for buffer positions etc.

* coding.c (detect_coding_utf_8, emacs_mule_char)
(detect_coding_iso_2022, encode_coding_iso_2022, check_ascii)
(check_utf_8, decode_coding):
* coding.h (struct coding_system.errors):
Use ptrdiff_t, not int, for integer values derived from buffer and
string positions.

src/coding.h

@@ -477,7 +477,7 @@ struct coding_system
   ptrdiff_t produced, produced_char, consumed, consumed_char;
 
   /* Number of error source data found in a decoding routine.  */
-  int errors;
+  ptrdiff_t errors;
 
   /* Store the positions of error source data.  */
   ptrdiff_t *error_positions;