perf probe: Check if *ptr2 is zero and not ptr2

perf probe: Check if *ptr2 is zero and not ptr2

Static anaylsis with cppcheck[1] detected an incorrect comparison:
[tools/perf/util/probe-event.c:216]: (warning) Char literal compared
with pointer 'ptr2'. Did you intend to dereference it?

Dereference ptr2 for the comparison to fix this.

1: https://sourceforge.net/p/cppcheck/wiki/Home/

Signed-off-by: Colin King <colin.king@canonical.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 35726d3a4ca9 ("perf probe: Fix to cut off incompatible chars from group name")
Link: http://lkml.kernel.org/r/20161003103431.18534-1-colin.king@canonical.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>