Fix _lv_txt_ins with zero length insert text

Closes #1754

src/lv_misc/lv_txt.c

@@ -436,6 +436,8 @@ void _lv_txt_ins(char * txt_buf, uint32_t pos, const char * ins_txt)
 {
     size_t old_len = strlen(txt_buf);
     size_t ins_len = strlen(ins_txt);
+    if(ins_len == 0) return;
+
     size_t new_len = ins_len + old_len;
     pos              = _lv_txt_encoded_get_byte_id(txt_buf, pos); /*Convert to byte index instead of letter index*/