Fix #1660 (#1662)

Before the PR, espeak-ng -v it "0 items" caused a segmentation fault.

- test for #1660
- Revert "cleanup: fix code smell knownConditionTrueFalse for strlen()
>= 0"

src/libespeak-ng/numbers.c

 				sprintf(ph_out, "%s%s%s%s", ph_tens, ph_and, ph_digits, ph_ordinal);
 			used_and = 1;
 		} else {
-			if ((tr->langopts.numbers & NUM_SINGLE_VOWEL) && ph_digits[0] != 0) {
+			if (tr->langopts.numbers & NUM_SINGLE_VOWEL) {
 				// remove vowel from the end of tens if units starts with a vowel (LANG=Italian)
-				ix = strlen(ph_tens) - 1;
-				if ((next_phtype = phoneme_tab[(unsigned int)(ph_digits[0])]->type) == phSTRESS)
-					next_phtype = phoneme_tab[(unsigned int)(ph_digits[1])]->type;
+				if (((ix = strlen(ph_tens)-1) >= 0) && (ph_digits[0] != 0)) {
+					if ((next_phtype = phoneme_tab[(unsigned int)(ph_digits[0])]->type) == phSTRESS)
+						next_phtype = phoneme_tab[(unsigned int)(ph_digits[1])]->type;
 
-				if ((phoneme_tab[(unsigned int)(ph_tens[ix])]->type == phVOWEL) && (next_phtype == phVOWEL))
-					ph_tens[ix] = 0;
+					if ((phoneme_tab[(unsigned int)(ph_tens[ix])]->type == phVOWEL) && (next_phtype == phVOWEL))
+						ph_tens[ix] = 0;
+				}
 			}
 
 			if ((tr->langopts.numbers2 & NUM2_ORDINAL_DROP_VOWEL) && (ph_ordinal[0] != 0)) {

tests/language-numbers-cardinal.test

 test_phon lv "s'imts s'imts v'iens d'ivi s'imti d'ivi tR'i:s s'imti s'eSdesmit p'ietsi" "100 101 202 365" "hundreds"
 test_phon lv "s'imts t'u:kstuo:tis m'iljo:ns m'iljaRds tR'iljons kv'adRiljo:ns kv'intiljo:ns s'ekstiljo:ns s'eptiljo:ns 'oktiljo:ns n'oniljo:ns" "100 1000 1000000 1000000000 1000000000000 1000000000000000 1000000000000000000 1000000000000000000000 1000000000000000000000000 1000000000000000000000000000 1000000000000000000000000000000" "large" # 10^2 10^3 10^6 10^9 10^12 10^15 10^18 10^21 10^24 10^27 10^30
 test_phon lv "p'ietsi s'eSdesmit tS'etRi s'imts d'ivdesmit tR'i:s tS'etRi t'u:kstuo:Si tR'i:s s'imti p'ietsd,esmit s'eptin^i tS'etRpadsmit t'u:kstuo:Si_! s'eSi s'imti_ 'astuon^desmit tR'i:s s'eptin^i s'imti tS'etRdesmit d'ivi t'u:kstuo:Si_! tR'i:s s'imti p'ietsd,esmit d'evin^i s'eSi m'iljo:ni_! tS'etRi s'imti s'eptin^desmit p'ietsi t'u:kstuo:Si_! tR'i:s s'imti d'ivdesmit 'astuon^i" "5 64 123 4357 14683 742359 6475328" "mixed"
+test_phon it "dz'E*o_| 'items" "0 items" "NUM_SINGLE_VOWEL"