[3.15] gh-151842: Fix crash upon OOM in _interpreters.capture_exception (GH-151843) (GH-152031)

(cherry picked from commit 5e0747d)

Co-authored-by: Amrutha <amruthamodela06@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Library/2026-06-30-13-00-00.gh-issue-151842.OOM31g.rst

@@ -0,0 +1,2 @@
+Fix a crash in :func:`!_interpreters.capture_exception` when
+:exc:`MemoryError` happens. Patch by Amrutha Modela.

Modules/_interpretersmodule.c

@@ -1541,7 +1541,9 @@ _interpreters_capture_exception_impl(PyObject *module, PyObject *exc_arg)
     }
 
 finally:
-    _PyXI_FreeExcInfo(info);
+    if (info != NULL) {
+        _PyXI_FreeExcInfo(info);
+    }
     if (exc != exc_arg) {
         if (PyErr_Occurred()) {
             PyErr_SetRaisedException(exc);

Python/crossinterp.c

@@ -1689,6 +1689,7 @@ _PyXI_NewExcInfo(PyObject *exc)
     }
     _PyXI_excinfo *info = PyMem_RawCalloc(1, sizeof(_PyXI_excinfo));
     if (info == NULL) {
+        PyErr_NoMemory();
         return NULL;
     }
     const char *failure;
@@ -1709,6 +1710,7 @@ _PyXI_NewExcInfo(PyObject *exc)
 void
 _PyXI_FreeExcInfo(_PyXI_excinfo *info)
 {
+    assert(info != NULL);
     _PyXI_excinfo_clear(info);
     PyMem_RawFree(info);
 }