MatchRule: Do not go back before the pre-ptr

pre_ptr is already one byte before the current letter, so we do not want
to subtract 1 again. Otherwise this would for instance underflow word_iz
of addPluralSuffixes.

src/libespeak-ng/dictionary.c

 					break;
 				case RULE_NOVOWELS:
 				{
-					char *p = pre_ptr - letter_xbytes - 1;
+					char *p = pre_ptr - letter_xbytes;
 					while (letter_w != RULE_SPACE) {
 						if (IsLetter(tr, letter_w, LETTERGP_VOWEL2)) {
 							failed = 1;