gh-153569: fix multiline continuation error offsets

Author: pablogsal

Lib/test/test_syntax.py

@@ -3393,6 +3393,10 @@ def test_invalid_line_continuation_error_position(self):
         self._check_error('\nfgdfgf\n1,\\#\n2\n',
                           "unexpected character after line continuation character",
                           lineno=3, offset=4)
+        for prefix in ("f", "t"):
+            self._check_error(f'{prefix}"""{{\n\\ x}}"""',
+                              "unexpected character after line continuation character",
+                              lineno=2, offset=2)
 
     def test_invalid_line_continuation_left_recursive(self):
         # Check bpo-42218: SyntaxErrors following left-recursive rules

Parser/pegen_errors.c

@@ -63,7 +63,7 @@ _Pypegen_tokenizer_error(Parser *p)
             msg = "too many levels of indentation";
             break;
         case E_LINECONT: {
-            col_offset = p->tok->cur - p->tok->buf - 1;
+            col_offset = p->tok->cur - p->tok->line_start - 1;
             msg = "unexpected character after line continuation character";
             break;
         }