bpo-29854: Fix segfault in call_readline()

If history-length is set in .inputrc, and the history file is double the
history size (or more), history_get(N) returns NULL, and python
segfaults. Fix that by checking for NULL return value.

It seems that the root cause is incorrect handling of bigger history in
readline, but python should not segfault even if readline returns
unexpected value.

Author: nirs

Lib/test/test_readline.py

@@ -212,7 +212,6 @@ def display(substitution, matches, longest_match_length):
 
     @unittest.skipIf(is_editline,
                      "editline history size configuration is broken")
-    @unittest.expectedFailure
     def test_history_size(self):
         history_size = 10
         with temp_dir() as test_dir:

Misc/NEWS.d/next/Library/2017-07-07-02-18-57.bpo-29854.J8wKb_.rst

@@ -0,0 +1,2 @@
+Fix segfault in readline when using readline's history-size option.  Patch
+by Nir Soffer.

Modules/readline.c

@@ -1347,15 +1347,17 @@ call_readline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
     if (should_auto_add_history && n > 0) {
         const char *line;
         int length = _py_get_history_length();
-        if (length > 0)
+        if (length > 0) {
+            HIST_ENTRY *hist_ent;
 #ifdef __APPLE__
             if (using_libedit_emulation) {
                 /* handle older 0-based or newer 1-based indexing */
-                line = (const char *)history_get(length + libedit_history_start - 1)->line;
+                hist_ent = history_get(length + libedit_history_start - 1);
             } else
 #endif /* __APPLE__ */
-            line = (const char *)history_get(length)->line;
-        else
+                hist_ent = history_get(length);
+            line = hist_ent ? hist_ent->line : "";
+        } else
             line = "";
         if (strcmp(p, line))
             add_history(p);