Fix calling utf8_in2

utf8_in2, when working in backward mode, is assuming that we are giving
the address of the last byte of the previous character (see all other
calls to utf8_in2). Otherwise, utf8_in2 returns the size of the current
multibyte character instead of that of the previous multibyte character.

src/libespeak-ng/dictionary.c

 							failed = 1;
 							break;
 						}
-						p -= utf8_in2(&letter_w, p, 1);
+						p -= utf8_in2(&letter_w, p-1, 1);
 					}
 					if (!failed)
 						add_points = 3;