Consolidate "if"s in tui_show_frame_info

Consolidate "if"s in tui_show_frame_info

tui_show_frame_info has two parallel "if"s with the same condition and
no intervening code.  The second "if" perhaps wasn't obviously
identical, but earlier refactorings made it clearer, by pointing out
that only source and disassembly windows can be handled here.  This
merges the two statements.

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

	* tui/tui-stack.c (tui_show_frame_info): Consolidate "if"s.