Fix use-of-uninitialized-value

84e967c0a2 ("Update translate.c") (sic) dropped the initialization of
number_buf, breaking the CI on ubuntu amd64 clang+msan, ubuntu amd64/i386
gcc/clang+valgrind,

While this probably deserves cleaning up, we'd rather not break the CI,
to be able to properly track regressions.

src/libespeak-ng/translate.c

 			if (dict_flags & FLAG_SPELLWORD) {
 				// redo the word, speaking single letters
 				for (pw = word; *pw != ' ';) {
+					memset(number_buf, 0, sizeof(number_buf));
 					memset(number_buf, ' ', 9);
 					nx = utf8_in(&c_temp, pw);
 					memcpy(&number_buf[2], pw, nx);