Simplify the !v comparison and check PATHSEP

SetVoiceStack looks for "!v" in variant_name and skips the first
three characters if "!v" is found. The problem here is that it
does not check that the third character is the path separator, so
may advance into unknown memory if variant_name is exactly "!v".

This fixes that problem by checking for the path separator. It
also simplifies the logic by checking the bytes explicitly.

NOTE: This is not strictly needed, as the only code paths this is
relevant for is in espeak_ng_SetVoiceByName, and the variant name
comes from ExtractVoiceVariantName, which sets up the variant name
correctly.

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 (strlen(variant_name) >= 2 && memcmp(variant_name, "!v", 2) == 0)
+	if (variant_name[0] == '!' && variant_name[1] == 'v' && variant_name[2] == PATHSEP)
 		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));