perf kvm: Initialize file_name var to fix segfault

perf kvm: Initialize file_name var to fix segfault

The 3786063 commit:

    perf kvm: Rename perf_kvm to perf_kvm_stat

Moved the file_name from inside a local struct var that initialized some
of its members, thus zero initializing the not explicitely initialized
variables, one of which was 'file_name', to a standalone local variable,
but forgot to initialize it explicitely to NULL, so it then got some
undefined value, causing a segfault in strdup when it wasn't, by luck,
zero.

Fix it by explicitely initializing it to NULL.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/n/tip-qo2jevp1bdcnh8khzdazs17s@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>