gdb: add assert in cmd_list_element::set_context

gdb: add assert in cmd_list_element::set_context

If something tries to set a context pointer on a cmd_list_element and
m_context is not nullptr, it's likely that two parts of the code are
trying to set different contexts, and one will overwrite the other.
This is almost guaranteed to lead to bad behavior or a crash, as one of
the spots will not be using the data it expects.  This happened to me
during development, so I think having this assert would be useful to
catch this problem earlier.

gdb/ChangeLog:

	* cli/cli-decode.h (struct cmd_list_element) <set_context>: Add
	assert.

Change-Id: I1f2e9fda1bf2bec1b732c9b90e7d7910a97f2ac6