MDEV-19670 json escaped unicode parse error.

Fixed 4-byte length characters handled incorrectly.

mysql-test/main/func_json.result

@@ -942,5 +942,14 @@ def					json_depnth	3	10	1	N	32897	0	63
 json_length	json_depnth
 2	3
 #
+# MDEV-19670 json escaped unicode parse error
+#
+SELECT json_valid('{"value":"\\ud83d\\ude0a"}');
+json_valid('{"value":"\\ud83d\\ude0a"}')
+1
+SELECT json_valid('{"test": "\\ud83d\\ude0b"}');
+json_valid('{"test": "\\ud83d\\ude0b"}')
+1
+#
 # End of 10.3 tests
 #

mysql-test/main/func_json.test

@@ -561,6 +561,13 @@ SELECT
 --enable_ps_protocol
 --disable_metadata
 
+--echo #
+--echo # MDEV-19670 json escaped unicode parse error
+--echo #
+
+SELECT json_valid('{"value":"\\ud83d\\ude0a"}');
+SELECT json_valid('{"test": "\\ud83d\\ude0b"}');
+
 --echo #
 --echo # End of 10.3 tests
 --echo #

strings/json_lib.c

@@ -320,15 +320,17 @@ static int json_handle_esc(json_string_t *s)
     if (s->c_next != '\\')
       return s->error= JE_SYN;
 
+    s->c_str+= c_len;
     if ((c_len= json_next_char(s)) <= 0)
       return s->error= json_eos(s) ? JE_EOS : JE_BAD_CHR;
     if (s->c_next != 'u')
       return s->error= JE_SYN;
+    s->c_str+= c_len;
 
     if (read_4_hexdigits(s, code+2))
       return 1;
 
-    if ((c_len= my_utf16_uni(0, &s->c_next, code, code+4)) == 2)
+    if ((c_len= my_utf16_uni(0, &s->c_next, code, code+4)) == 4)
       return 0;
   }
   return s->error= JE_BAD_CHR;