[3.15] gh-155504: Set the interpreter before fallible thread state initialization (GH-155590) (GH-155884)

(cherry picked from commit 94a75f7)

Co-authored-by: Abdul Mohammad <154353563+abdulm5@users.noreply.github.com>

Author: miss-islington

Misc/NEWS.d/next/Core_and_Builtins/2026-08-11-13-50-35.gh-issue-155504.PsTqZm.rst

@@ -0,0 +1,2 @@
+Fix a crash when thread state creation fails before the thread state is fully
+initialized.

Python/pystate.c

@@ -1479,6 +1479,8 @@ alloc_threadstate(PyInterpreterState *interp)
         }
         reset_threadstate(tstate);
     }
+    // Set the interpreter before any later initialization can fail.
+    tstate->base.interp = interp;
     return tstate;
 }