[RFA] Initialize py_type_printers in ext_lang_type_printers

[Tom Tromey <tom@tromey.com>]

When running gdb in the build directory without passing
--data-directory, I noticed I could provoke a crash by:

    $ ./gdb -nx ./gdb
    (gdb) ptype/o struct dwarf2_per_objfile

... and then trying to "q" out at the pagination prompt.

valgrind complained about an uninitialized use of py_type_printers.
Initializing this member fixes the bug.

I believe this bug can occur even when the gdb Python libraries are
available, for example if get_type_recognizers fails.

Tested by hand on x86-64 Fedora 26.  No test case because it seemed
difficult to guarantee failures.

gdb/ChangeLog
2018-05-16  Tom Tromey  <tom@tromey.com>

	* extension.h (struct ext_lang_type_printers) <py_type_printers>:
	Initialize.
---
 gdb/ChangeLog   | 5 +++++
 gdb/extension.h | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/gdb/extension.h b/gdb/extension.h
index abde5e54e1..0c8c4ee934 100644
--- a/gdb/extension.h
+++ b/gdb/extension.h
@@ -152,7 +152,7 @@ struct ext_lang_type_printers
   DISABLE_COPY_AND_ASSIGN (ext_lang_type_printers);
 
   /* Type-printers from Python.  */
-  void *py_type_printers;
+  void *py_type_printers = nullptr;
 };
 
 /* The return code for some API calls.  */
-- 
2.13.6