Compare variant_name with "!v" only if long enough

Various places call SetVoiceStack with "" for the variant_name. This
causes -fsanitize=address to fail with an overflow as the call to
memcmp is checking the first 2 bytes, and there is only 1 byte
available.

src/libespeak-ng/readclause.c

@@ -599,7 +599,7 @@ void SetVoiceStack(espeak_VOICE *v, const char *variant_name)
 	sp->voice_age = v->age;
 	sp->voice_gender = v->gender;
 
-	if (memcmp(variant_name, "!v", 2) == 0)
+	if (strlen(variant_name) >= 2 && memcmp(variant_name, "!v", 2) == 0)
 		variant_name += 3; // strip variant directory name, !v plus PATHSEP
 	strncpy0(base_voice_variant_name, variant_name, sizeof(base_voice_variant_name));
 	memcpy(&base_voice, &current_voice_selected, sizeof(base_voice));