Fix Py_DECREF being executed without holding the GIL

Fix Py_DECREF being executed without holding the GIL

When the gdbpy_ref objects get destroyed, they call Py_DECREF to
decrement the reference counter of the python object they hold a
reference to.  Any time we call into the Python API, we should be
holding the GIL.  The gdbpy_enter object does that for us in an
RAII-fashion.

However, if gdbpy_enter is declared after a gdbpy_ref object in a
function, gdbpy_enter's destructor will be called (and the GIL will be
released) before gdbpy_ref's destructor is called.  Therefore, we will
end up calling Py_DECREF without holding the GIL.

This became obvious with Python 3.6, where memory management functions
have asserts to make sure that the GIL is held.  This was exposed by
tests py-as-string.exp, py-function.exp and py-xmethods.  For example:

  (gdb) p $_as_string(enum_valid)
  Fatal Python error: Python memory allocator called without holding the GIL

  Current thread 0x00007f7f7b21c780 (most recent call first):
  [1]    18678 abort (core dumped)  ./gdb -nx testsuite/outputs/gdb.python/py-as-string/py-as-string

  #0  0x00007ffff618bc37 in raise () from /lib/x86_64-linux-gnu/libc.so.6
  #1  0x00007ffff618f028 in abort () from /lib/x86_64-linux-gnu/libc.so.6
  #2  0x00007ffff6b104d6 in Py_FatalError (msg=msg@entry=0x7ffff6ba15b8 "Python memory allocator called without holding the GIL") at Python/pylifecycle.c:1457
  #3  0x00007ffff6a37a68 in _PyMem_DebugCheckGIL () at Objects/obmalloc.c:1972
  #4  0x00007ffff6a3804e in _PyMem_DebugFree (ctx=0x7ffff6e65290 <_PyMem_Debug+48>, ptr=0x24f8830) at Objects/obmalloc.c:1994
  #5  0x00007ffff6a38e1d in PyMem_Free (ptr=<optimized out>) at Objects/obmalloc.c:442
  #6  0x00007ffff6b866c6 in _PyFaulthandler_Fini () at ./Modules/faulthandler.c:1369
  #7  0x00007ffff6b104bd in Py_FatalError (msg=msg@entry=0x7ffff6ba15b8 "Python memory allocator called without holding the GIL") at Python/pylifecycle.c:1431
  #8  0x00007ffff6a37a68 in _PyMem_DebugCheckGIL () at Objects/obmalloc.c:1972
  #9  0x00007ffff6a3804e in _PyMem_DebugFree (ctx=0x7ffff6e652c0 <_PyMem_Debug+96>, ptr=0x7ffff46b6040) at Objects/obmalloc.c:1994
  #10 0x00007ffff6a38f55 in PyObject_Free (ptr=<optimized out>) at Objects/obmalloc.c:503
  #11 0x00007ffff6a5f27e in unicode_dealloc (unicode=unicode@entry=0x7ffff46b6040) at Objects/unicodeobject.c:1794
  #12 0x00007ffff6a352a9 in _Py_Dealloc (op=0x7ffff46b6040) at Objects/object.c:1786
  #13 0x000000000063f28b in gdb_Py_DECREF (op=0x7ffff46b6040) at /home/emaisin/src/binutils-gdb/gdb/python/python-internal.h:192
  #14 0x000000000063fa33 in gdbpy_ref_policy::decref (ptr=0x7ffff46b6040) at /home/emaisin/src/binutils-gdb/gdb/python/py-ref.h:35
  #15 0x000000000063fa77 in gdb::ref_ptr<_object, gdbpy_ref_policy>::~ref_ptr (this=0x7fffffffcdf0, __in_chrg=<optimized out>) at /home/emaisin/src/binutils-gdb/gdb/common/gdb_ref_ptr.h:91
  #16 0x000000000064d8b8 in fnpy_call (gdbarch=0x2b50010, language=0x115d2c0 <c_language_defn>, cookie=0x7ffff46b7468, argc=1, argv=0x7fffffffcf48)
    at /home/emaisin/src/binutils-gdb/gdb/python/py-function.c:145

The fix is to place the gdbpy_enter first in the function.  I also
cleaned up the comments a bit and removed the unnecessary initialization
of the value variable.

gdb/ChangeLog:

	* python/py-function.c (fnpy_call): Reorder declarations to have
	the gdbpy_enter object declared first.
	* python/py-xmethods.c (gdbpy_get_xmethod_arg_types): Likewise.