tui-disasm: Fix window content buffer overrun

tui-disasm: Fix window content buffer overrun

A user reported a GDB crash with TUI when trying to debug a function
with a long demangled C++ method name.  It turned out that the logic for
displaying the TUI disassembly window has a bug that can cause a buffer
overrun, possibly overwriting GDB-internal data structures.  In
particular, the logic performs an unguarded strcpy.

Another (harmless) bug in tui_alloc_source_buffer causes the buffer to
be two lines longer than needed.  This may have made the crash appear
less frequently.

gdb/ChangeLog:

	* tui/tui-disasm.c (tui_set_disassem_content): Fix line buffer
	overrun due to unchecked strcpy.

gdb/testsuite/ChangeLog:

	* gdb.base/tui-layout.c: New file.
	* gdb.base/tui-layout.exp: Use tui-layout.c, to ensure that the
	disassembly window contains very long lines.