gdb/tui: Disassembler scrolling of very small programs

gdb/tui: Disassembler scrolling of very small programs

In TUI mode, if the disassembly output for the program is less than
one screen long, then currently if the user scrolls down until on the
last assembly instruction is displayed and then tries to scroll up
using Page-Up, the display doesn't update - they are stuck viewing the
last line.

If the user tries to scroll up using the Up-Arrow, then the display
scrolls normally.

What is happening is on the Page-Up we ask GDB to scroll backward the
same number of lines as the height of the TUI ASM window.  The back
scanner, which looks for a good place to start disassembling, fails to
find a starting address which will provide the requested number of new
lines before we get back to the original starting address (which is
not surprising, our whole program contains less than a screen height
of instructions), as a result the back scanner gives up and returns
the original starting address.

When we scroll with Up-Arrow we only ask the back scanner to find 1
new instruction, which it manages to do, so this scroll works.

The solution here is, when we fail to find enough instructions, to
return the lowest address we did manage to find.  This will ensure we
jump to the lowest possible address in the disassembly output.

gdb/ChangeLog:

	PR tui/9765
	* tui/tui-disasm.c (tui_find_disassembly_address): If we don't
	have enough lines to fill the screen, still return the lowest
	address we found.

gdb/testsuite/ChangeLog:

	PR tui/9765
	* gdb.tui/tui-layout-asm-short-prog.S: New file.
	* gdb.tui/tui-layout-asm-short-prog.exp: New file.

Change-Id: I6a6a7972c68a0559e9717fd8d82870b669a40af3