Work around a regression in Windows 7, causing erase_in_line() to crash sometimes

Work around a regression in Windows 7, causing erase_in_line() to crash sometimes

The function FillConsoleOutputCharacterA() was pretty content in XP to take a NULL
pointer if we did not want to store the number of written columns.  In Windows 7,
it crashes, but only when called from within Git Bash, not from within cmd.exe.
Go figure.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>