Fix assertion in UndoListView

No longer pass -1 to wxListCtrl::EnsureVisible() function in order to fix the following assertion:
src/generic/listctrl.cpp(4072): assert "index >= 0 && (size_t)index < GetItemCount()" failed in EnsureVisible(): invalid index in EnsureVisible
Fix compilation warning in WolfArchive.cpp

Clang reported the following warning for findFileCasing() function:
control may reach end of non-void function [-Wreturn-type]
Fix compilation warning and potential memory leak in MapCheck class

Clang reported the following warning for any call to delete operator for instance of class derived from MapCheck:
delete called on 'MapCheck' that is abstract but has non-virtual destructor [-Wdelete-non-virtual-dtor]
Fix compilation warning in SetupWizardDialog

Clang reported the following warning for SetupWizardDialog::showPage() function:
comparison of unsigned expression < 0 is always false [-Wtautological-compare]
Fix compilation warning in ArchiveEntry

Clang reported the following warning for ArchiveEntry::importFile() function:
comparison of unsigned expression < 0 is always false [-Wtautological-compare]
Fix compilation warning in ArchivePanel

Clang reported the following warning for ArchivePanel::newEntry() function:
warning: '&&' within '||' [-Wlogical-op-parentheses]
Fix compilation warning in CSoundFile

Clang reported the following warning for CSoundFile::ReadAMF() function:
'memset' call operates on objects of type 'BYTE *' (aka 'unsigned char *') while the size is based on a different type 'BYTE **' (aka 'unsigned char **') [-Wsizeof-pointer-memaccess]
Merge pull request #138 from alexey-lysiuk/small-fixes

Several small fixes
Merge branch 'master' of https://github.com/sirjuddington/SLADE into osx