gh-93418: Fix an assert when an f-string expression is followed by an '=', but no closing brace. (gh-93419)

(cherry picked from commit ee70c70)

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>

Author: ericvsmith

Lib/test/test_fstring.py

@@ -1073,6 +1073,7 @@ def test_mismatched_braces(self):
                              "f'{'",
                              "f'x{<'",  # See bpo-46762.
                              "f'x{>'",
+                             "f'{i='",  # See gh-93418.
                              ])
 
         # But these are just normal strings.

Misc/NEWS.d/next/Core and Builtins/2022-06-01-17-47-40.gh-issue-93418.24dJuc.rst

@@ -0,0 +1,2 @@
+Fixed an assert where an f-string has an equal sign '=' following an
+expression, but there's no trailing brace. For example, f"{i=".

Parser/string_parser.c

@@ -756,7 +756,9 @@ fstring_find_expr(Parser *p, const char **str, const char *end, int raw, int rec
         while (Py_ISSPACE(**str)) {
             *str += 1;
         }
-
+        if (*str >= end) {
+            goto unexpected_end_of_string;
+        }
         /* Set *expr_text to the text of the expression. */
         *expr_text = PyUnicode_FromStringAndSize(expr_start, *str-expr_start);
         if (!*expr_text) {