perf stat: Print generic metric header even for failed expressions

Print the generic metric header even when the expression evaluation
failed. Otherwise an expression that fails on the first collections due
to division by zero may suddenly reappear later without an header.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170831194036.30146-5-andi@firstfloor.org


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