Add more heading margin in TranslateClause

Valgrind reports:

==3642987== Conditional jump or move depends on uninitialised value(s)
==3642987==    at 0x491F268: TranslateNumber_1 (numbers.c:1785)
==3642987==    by 0x4923C35: TranslateNumber (numbers.c:2080)
==3642987==    by 0x49556DC: TranslateWord3 (translate.c:644)
==3642987==    by 0x4957FCE: TranslateWord (translate.c:1100)
==3642987==    by 0x4959344: TranslateWord2 (translate.c:1361)
==3642987==    by 0x496116E: TranslateClause (translate.c:2613)
==3642987==    by 0x494FF7A: SpeakNextClause (synthesize.c:1569)
==3642987==    by 0x4939B9D: Synthesize (speech.c:457)
==3642987==    by 0x493AE6A: sync_espeak_Synth (speech.c:570)
==3642987==    by 0x493B286: espeak_ng_Synthesize (speech.c:678)
==3642987==    by 0x4916925: espeak_Synth (espeak_api.c:90)
==3642987==    by 0x10CF5D: main (espeak-ng.c:691)
==3642987==  Uninitialised value was created by a stack allocation
==3642987==    at 0x495BD9F: TranslateClause (translate.c:1941)

Indeed, TranslateNumber_1 looks back up to three bytes before, with
IsDigit09(word[-3])), so we have to increase the heading margin to three
spaces.

src/libespeak-ng/translate.c

 		if (n_digits > 4) {
 			// word is entirely digits, insert commas and break into 3 digit "words"
 			number_buf[0] = ' ';
-			pn = &number_buf[1];
+			number_buf[1] = ' ';
+			number_buf[2] = ' ';
+			pn = &number_buf[3];
 			nx = n_digits;
 			nw = 0;
 
 			pn[16] = 0;
 			nw = 0;
 
-			for (pw = &number_buf[1]; pw < pn;) {
+			for (pw = &number_buf[3]; pw < pn;) {
 				// keep wflags for each part, for FLAG_HYPHEN_AFTER
 				dict_flags = TranslateWord2(tr, pw, &num_wtab[nw++], words[ix].pre_pause);
 				while (*pw++ != ' ')