COMP: Eliminate "conversion may change sign of result" warnings by using...

COMP: Eliminate "conversion may change sign of result" warnings by using size_t where appropriate. (Missed one warning with last commit: add a cast to md5_word_t.)

MD5.c

@@ -382,7 +382,7 @@ static void md5_append(md5_state_t *pms, const md5_byte_t *data, size_t nbytes)
         return;
 
     /* Update the message length. */
-    pms->count[1] += nbytes >> 29;
+    pms->count[1] += (md5_word_t)(nbytes >> 29);
     pms->count[0] += nbits;
     if (pms->count[0] < nbits)
         pms->count[1]++;