gh-159739: Check character formatting allocation failure

Propagate the error from PyUnicode_FromOrdinal() before using the
result.

Backport to 3.15 and 3.14. On 3.14, the equivalent code is in
Python/formatter_unicode.c.

Author: JelleZijlstra

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

@@ -1258,6 +1258,9 @@ 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);