Fix off-by-one line number in parse error messages

cursor_position consumed the newline ending the previous line (post-decrement)
before counting lines, so any error past the first line was reported one line
too low (e.g. "[1,\n@" reported '@' at line 1 instead of line 2). Count that
newline when the column loop breaks on it.

Co-Authored-By: Jean Boussier <jean.boussier@gmail.com>

Author: mame

ext/json/ext/parser/parser.c

@@ -593,6 +593,7 @@ static void cursor_position(JSON_ParserState *state, long *line_out, long *colum
 
     while (cursor >= state->start) {
         if (*cursor-- == '\n') {
+            line++;
             break;
         }
         column++;

test/json/json_parser_test.rb

@@ -862,7 +862,7 @@ def test_parse_error_incomplete_hash
   end
 
   def test_parse_error_snippet
-    omit "C ext only test" unless RUBY_ENGINE == "ruby"
+    omit "JRuby errors don't contain positions" unless RUBY_ENGINE == "ruby"
 
     error = assert_raise(JSON::ParserError) { JSON.parse("あああああああああああああああああああああああ") }
     assert_equal "unexpected character: 'ああああああああああ' at line 1 column 1", error.message
@@ -875,6 +875,15 @@ def test_parse_error_snippet
 
     error = assert_raise(JSON::ParserError) { JSON.parse("abcあああああああああああああああああああああああ") }
     assert_equal "unexpected character: 'abcあああああああああ' at line 1 column 1", error.message
+
+    error = assert_raise(JSON::ParserError) { JSON.parse("[1,\n@") }
+    assert_equal "unexpected character: '@' at line 2 column 1", error.message
+
+    error = assert_raise(JSON::ParserError) { JSON.parse("[\n  1,\n  @\n]") }
+    assert_equal "unexpected character: '@' at line 3 column 3", error.message
+
+    error = assert_raise(JSON::ParserError) { JSON.parse("@") }
+    assert_equal "unexpected character: '@' at line 1 column 1", error.message
   end
 
   def test_parse_leading_slash