DOS: fix loading of DOS exe files

[Michael Stefaniuc]

Hello,

the attached patch fixes a problem when loading DOS exe files which have
4 bytes on the last page of the executable. There is no comment in the
code and i googled and couldn't find any reason or hint why having 4
bytes on the last page is special and why we should than load the full
512 bytes of the last page (i'm CC'ing wine-devel just in case). This
prevents some of the "PC Intern 4, Systemprogrammierung" DOS example
files from running with an 'invalid exe file' error.

License: LGPL, X11
Changelog:
    Michael Stefaniuc 
    - do not load the full last page of a DOS EXE if we have only 4
      bytes in it.

bye
    michael