Change tui_alloc_source_buffer return type to void

Change tui_alloc_source_buffer return type to void

tui_alloc_source_buffer can't actually fail, so change its return type
to void and update the callers.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (tui_alloc_source_buffer): Change return
	type to void.
	* tui/tui-winsource.c (tui_alloc_source_buffer): Change return
	type to void.
	* tui/tui-source.c (tui_set_source_content): Update.
	* tui/tui-disasm.c (tui_set_disassem_content): Update.