git: predictible crash when chaging colors in mintty

When running git ( Git-2.9.3.2-32-bit.exe) at r72556, a predictible crash
occurs if you try to change the colors in mintty.

Steps to reproduce:
1) install Git-2.9.3.2-32-bit.exe
2) start the bash shell
3) right-click in the window to open the options dialogs, and select a forground and a background color
4) press OK

I'm submitting a screenshot just before the crash, and a full debug log with backtrace.

I've already reported this at CORE-11904 but HBelusca told me to create a separate ticket for this issue.