[3.13] gh-150372: Add missing null check on completer_word_break_characters in readline.c (GH-150251) (GH-150630)

(cherry picked from commit 56bd9ea)

Co-authored-by: Thomas Kowalski <thom.kowa@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Library/2026-05-25-07-22-05.gh-issue-150372.9hLqhe.rst

@@ -0,0 +1,2 @@
+:mod:`readline`: Fix a potential crash during tab completion caused by an
+out-of-memory error during module initialization.

Modules/readline.c

@@ -1318,6 +1318,10 @@ setup_readline(readlinestate *mod_state)
     completer_word_break_characters =
         strdup(" \t\n`~!@#$%^&*()-=+[{]}\\|;:'\",<>/?");
         /* All nonalphanums except '.' */
+
+    if (!completer_word_break_characters) {
+        goto error;
+    }
 #ifdef WITH_EDITLINE
     // libedit uses rl_basic_word_break_characters instead of
     // rl_completer_word_break_characters as complete delimiter
@@ -1361,6 +1365,10 @@ setup_readline(readlinestate *mod_state)
 
     RESTORE_LOCALE(saved_locale)
     return 0;
+
+error:
+    RESTORE_LOCALE(saved_locale)
+    return -1;
 }
 
 /* Wrapper around GNU readline that handles signals differently. */