Make gdb.PendingFrame.read_register handle "user" registers.

The C function, pending_framepy_read_register(), which implements
the python interface gdb.PendingFrame.read_register does not handle
the so called "user" registers like "pc".  An assertion error is
triggered due to the user registers having numbers larger than or
equal to gdbarch_num_regs(gdbarch).

With the VALUE_FRAME_ID tweak in place, the call to
get_frame_register_value() can simply be replaced by a call to
value_of_register(), which handles both real registers as well as the
user registers.

gdb/ChangeLog:

	* python/py-unwind.c (pending_framepy_read_register): Use
	value_of_register() instead of get_frame_register_value().

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2016-11-16  Kevin Buettner  <kevinb@redhat.com>
+
+	* python/py-unwind.c (pending_framepy_read_register): Use
+	value_of_register() instead of get_frame_register_value().
+
 2016-11-16  Kevin Buettner  <kevinb@redhat.com>
 
 	* value.h (VALUE_FRAME_ID): Rename to VALUE_NEXT_FRAME_ID. Update

gdb/python/py-unwind.c

@@ -407,7 +407,12 @@ pending_framepy_read_register (PyObject *self, PyObject *args)
 
   TRY
     {
-      val = get_frame_register_value (pending_frame->frame_info, regnum);
+      /* Fetch the value associated with a register, whether it's
+	 a real register or a so called "user" register, like "pc",
+	 which maps to a real register.  In the past,
+	 get_frame_register_value() was used here, which did not
+	 handle the user register case.  */
+      val = value_of_register (regnum, pending_frame->frame_info);
       if (val == NULL)
         PyErr_Format (PyExc_ValueError,
                       "Cannot read register %d from frame.",