In GimpTool, always clear tool->drawable upon halting, even for
tools that don't use it explicitly.

GimpTool sets tool->drawable in its default button_press()
implementation, and we potentially access it in
gimp_display_shell_initialize_tool(), so failing to clear it when
halting the tool may leave it as a dangling pointer, which can
result in a segfault when trying to initialize the tool in the
above function.  In particular, this happens with the iscissors
tool.

(cherry picked from commit 58c96f59)