Fix use of uninitialized memory.

When creating an UnwindMapLocal fails in the Build() function call,
the destructor for UnwindMap is called. Unfortunately, the map_cursor_
member variable has not been initialized, so the call to destroy it
winds up operating on garbage data.

Part of this is a result of a bad class hierarchy, so this refactors
the classes slightly, and properly initializes the map_cursor_ member
variable in the base class.

Bug: 26931578
Change-Id: I885596bf65e4ef63559cee2c56cd41576d5ecc1b