Simplify show_source_disasm_command

Simplify show_source_disasm_command

This is the first of a few patches to further simplify window
(re-)initialization in tui-layout.c.  When changing the layout, a
window may be created or, if it already exists, simply resized.  These
two cases normally are identical, but this was obscured by the way the
code was written.  This patch changes show_source_disasm_command to
unify the creation and re-initialization cases.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (show_source_disasm_command): Simplify window
	resetting.