tokener: clear pending surrogate on reset

Discard the previous document's high surrogate when resetting the tokener.
Otherwise the next Unicode escape can add a replacement character or
combine with a surrogate from the abandoned document.

Extend the existing incremental parse tests for incomplete input and a
fatal error, using independent next-document string expectations.

Fixes #984.
Reported-by: GeekQAQ (GitHub)
Assisted-by: OpenAI Codex (AI)
Reviewed-by: GPT-6 Luna (AI)

json_tokener.c

@@ -214,6 +214,7 @@ void json_tokener_reset(struct json_tokener *tok)
 		json_tokener_reset_level(tok, i);
 	tok->depth = 0;
 	tok->err = json_tokener_success;
+	tok->high_surrogate = 0;
 }
 
 struct json_object *json_tokener_parse(const char *str)

tests/test_parse.c

@@ -335,6 +335,14 @@ struct incremental_step
     {"{ \"foo", -1, -1, json_tokener_continue, 1, 0},
     {": \"bar\"}", -1, 0, json_tokener_error_parse_unexpected, 1, 0},
 
+    /* Reset must discard a pending high surrogate from the previous document. */
+    {"\"\\uD834", -1, -1, json_tokener_continue, 1, 0},
+    {"\"\\u0041\"", -1, -1, json_tokener_success, 1, 0},
+    {"\"\\uD834", -1, -1, json_tokener_continue, 1, 0},
+    {"\"\\uDD1E\"", -1, -1, json_tokener_success, 1, 0},
+    {"\"\\uD834\\uZZZZ\"", -1, 9, json_tokener_error_parse_string, 1, 0},
+    {"\"\\u0041\"", -1, -1, json_tokener_success, 1, 0},
+
     /* Check a supplemental code point that looks like a high surrogate */
     {"\"\\uD836", -1, -1, json_tokener_continue, 0, 0},
     {"\\uDE87", -1, -1, json_tokener_continue, 0, 0},

tests/test_parse.expected

@@ -140,6 +140,12 @@ json_tokener_parse_ex(tok, "ä"        ,   4) ... OK: got object of type [string
 json_tokener_parse_ex(tok, "ä"        ,   4) ... OK: got object of type [string]: "ä"
 json_tokener_parse_ex(tok, { "foo      ,   6) ... OK: got correct error: continue
 json_tokener_parse_ex(tok, : "bar"}    ,   8) ... OK: got correct error: unexpected character
+json_tokener_parse_ex(tok, "\uD834     ,   7) ... OK: got correct error: continue
+json_tokener_parse_ex(tok, "\u0041"    ,   8) ... OK: got object of type [string]: "A"
+json_tokener_parse_ex(tok, "\uD834     ,   7) ... OK: got correct error: continue
+json_tokener_parse_ex(tok, "\uDD1E"    ,   8) ... OK: got object of type [string]: "�"
+json_tokener_parse_ex(tok, "\uD834\uZZZZ",  14) ... OK: got correct error: invalid string sequence
+json_tokener_parse_ex(tok, "\u0041"    ,   8) ... OK: got object of type [string]: "A"
 json_tokener_parse_ex(tok, "\uD836     ,   7) ... OK: got correct error: continue
 json_tokener_parse_ex(tok, \uDE87      ,   6) ... OK: got correct error: continue
 json_tokener_parse_ex(tok, "           ,   1) ... OK: got object of type [string]: "�"
@@ -388,5 +394,5 @@ json_tokener_parse_ex(tok, {"":1}     ,   7) ... OK: got correct error: invalid
 json_tokener_parse_ex(tok, {"":1}     ,   7) ... OK: got correct error: invalid string sequence
 json_tokener_parse_ex(tok, {"":1}     ,   7) ... OK: got correct error: invalid string sequence
 json_tokener_parse_ex(tok, {"":1}     ,   7) ... OK: got correct error: invalid string sequence
-End Incremental Tests OK=292 ERROR=0
+End Incremental Tests OK=298 ERROR=0
 ==================================