Fix MCDataAtom never calling remap when adding data.

This patch fixes a small mistake in MCDataAtom::addData() where it doesn't ever
call remap():

-  if (Data.size() > Begin - End - 1)
+  if (Data.size() > End + 1 - Begin)
     remap(Begin, End + 1);

This is currently not visible because of another bug is the disassembler, so
the patch includes a unit test.

Patch by Stephen Checkoway.