MatchRule: Avoid unbound loop reading with utf8_in (#2077)

src/libespeak-ng/dictionary.c

 						syllable_count += 1; // number of syllables to match
 					}
 					vowel = 0;
-					while (letter_w != RULE_SPACE) {
+					while (letter_w != RULE_SPACE && letter_w != 0) {
 						if ((vowel == 0) && IsLetter(tr, letter_w, LETTERGP_VOWEL2)) {
 							// this is counting vowels which are separated by non-vowel letters
 							vowel_count++;
 				case RULE_NOVOWELS:
 				{
 					char *p = post_ptr + letter_xbytes;
-					while (letter_w != RULE_SPACE) {
+					while (letter_w != RULE_SPACE && letter_w != 0) {
 						if (IsLetter(tr, letter_w, LETTERGP_VOWEL2)) {
 							failed = 1;
 							break;