[3.15] gh-159739: Check character formatting allocation failure (GH-159746) (#159839)

Propagate the error from PyUnicode_FromOrdinal() before using the
result.
(cherry picked from commit 5b0ab21)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>

Author: miss-islington

Misc/NEWS.d/next/Core_and_Builtins/2026-10-19-19-33-35.gh-issue-159739.173d49.rst

@@ -0,0 +1,2 @@
+Avoid a crash if formatting an integer with the ``c`` presentation type runs
+out of memory.

Objects/unicode_formatter.c

@@ -1256,6 +1256,9 @@ format_long_internal(PyObject *value, const InternalFormatSpec *format,
             goto done;
         }
         tmp = PyUnicode_FromOrdinal(x);
+        if (tmp == NULL) {
+            goto done;
+        }
         inumeric_chars = 0;
         n_digits = 1;
         maxchar = Py_MAX(maxchar, (Py_UCS4)x);