assert "IsOpened()" failed in GetFirst() when running in debug mode

[Sean Greenslade]

When compiling in debug mode and running from the build directory, an assertion is triggered due to the missing plugins directory.

ASSERT INFO:
./src/unix/dir.cpp(275): assert "IsOpened()" failed in GetFirst(): must wxDir::Open() first

BACKTRACE:
[1] wxDir::GetFirst(wxString*, wxString const&, int) const
[2] MainFrame::MainFrame(wxWindow*, HuginBase::Panorama&) /home/sean/pkg/hugin-hg/src/hugin/src/hugin1/hugin/MainFrame.cpp:371
[3] huginApp::OnInit() /home/sean/pkg/hugin-hg/src/hugin/src/hugin1/hugin/huginApp.cpp:315
[4] wxAppConsoleBase::CallOnInit() /usr/include/wx-3.0/wx/app.h:93
[5] wxEntry(int&, wchar_t**)
[6] main /home/sean/pkg/hugin-hg/src/hugin/src/hugin1/hugin/huginApp.cpp:118
[7] __libc_start_main
[8] _start

The attached patch fixes the issue by checking whether the dir open succeeded before attempting to access it. (I used hg diff this time, let me know if it's formatted right.)

[tmodes (tmodes)]

Thanks. Committed a slightly modified version.

We are using spaces for indentation, no tabs. (There are some place where tabs are still used. But I try to keep at spaces.)