perf thread-stack: Tidy thread_stack__push_cp() usage

If 'cp' is checked in thread_stack__push_cp() a number of error checks
can be removed, reducing code size and improving readability.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20190109091835.5570-3-adrian.hunter@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>