perf evlist: Explicititely initialize input_name

It was a global variable, so it was initialized, implicitely, to zero by
being placed in the bss.

Now it is just a local variable that is then passed to the __cmd_evlist
routine, so it must be explicitely set to NULL.

The problem manifested on a Fedora 17 system, using:

 gcc version 4.7.0 20120507 (Red Hat 4.7.0-5) (GCC)

But not on several other systems, by luck.

Reported-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.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>
Link: http://lkml.kernel.org/n/tip-5e8wolcjs3rgd5i6yi995gfh@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>