Handle size 5 or 6 resulting in stack smashing

src/utf8_fgetch.c

@@ -10,6 +10,12 @@ uint32_t utf8_fgetch(FILE *f) {
 	}
 	buffer[0] = (char)c;
 	int size = utf8_size(buffer);
+
+	if (size > UTF8_MAX_SIZE) {
+		fseek(f, size - 1, SEEK_CUR);
+		return UTF8_INVALID;
+	}
+
 	if (size > 1) {
 		int amt = fread(&buffer[1], 1, size - 1, f);
 		if (amt != size - 1) {