[2.7] bpo-29854: Fix segfault in call_readline() (GH-728)

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.

This issue affects only GNU readline. When using libedit emulation
system history size option does not work.

This is a backport of the actual fix from master without the test, since
the test depends on new run_pty() helper which is not available in 2.7.

Author: nirs

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

@@ -1161,15 +1161,17 @@ call_readline(FILE *sys_stdin, FILE *sys_stdout, char *prompt)
     if (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 = history_get(length + libedit_history_start - 1)->line;
+                hist_ent = history_get(length + libedit_history_start - 1);
             } else
 #endif /* __APPLE__ */
-            line = 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);