Fix reading hash table entries > 128.

This is related to ebfa320956169e3419234b72fee51bd596867661, but
when reading the hash chain entry length, not writing it.

If char is signed, then before this change the length would be
negative, causing problems loading the dictionary.

src/libespeak-ng/dictionary.c

 
 	for (hash = 0; hash < N_HASH_DICT; hash++) {
 		tr->dict_hashtab[hash] = p;
-		while ((length = *p) != 0)
+		while ((length = *(uint8_t *)p) != 0)
 			p += length;
 		p++; // skip over the zero which terminates the list for this hash value
 	}