maple_tree: fix mas_empty_area_rev() null pointer dereference

In the Linux kernel, the following vulnerability has been resolved:

maple_tree: fix mas_empty_area_rev() null pointer dereference

Currently the code calls mas_start() followed by mas_data_end() if the

maple state is MA_START, but mas_start() may return with the maple state

node == NULL. This will lead to a null pointer dereference when checking

information in the NULL node, which is done in mas_data_end().

Avoid setting the offset if there is no node by waiting until after the

maple state is checked for an empty or single entry state.

A user could trigger the events to cause a kernel oops by unmapping all

vmas to produce an empty maple tree, then mapping a vma that would cause

the scenario described above.