Added some security in scr-x

Preventing a crash when ScreenRestoreResolution is called without
a prior call to ScreenChangeResolution. Also avoiding a memory
leak when ScreenChangeResolution is called multiple times.

git-svn-id: svn+ssh://svn.code.sf.net/p/yabause/code/trunk@3238 d7be59e3-070e-4b9a-85a1-4646befae34d