ccache: Fix Segmentation fault error when gcc -o /dev/null

ccache: Fix Segmentation fault error when gcc -o /dev/null

Fixed:
$ export CCACHE_DEBUG=1
$ ccache gcc -c hello.c -o /dev/null

Segmentation fault (core dumped)

This is because failed to open /dev/null.foo (Permission denied), check file
stream before write to it can fix the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>