perf: Fix various display bugs with parent filtering

Hists that have been filtered, because they don't have callchains
matching the parent filter, won't be printed. As such,
hist_entry__snprintf() returns 0 for them, but we don't control
this value and we always print the buffer, which might be
untouched and then only made of random stack garbage.

Not only does it paint the screen with barf, it also prints
the callchains for these hists, even though they have been filtered,
since the hist has been filtered as well.

We need to check the return value of hist_entry__snprintf() and
ignore the hist if it is 0, which means it didn't get any callchain
matching the parent filter. This fixes the barf and the undesired
callchains.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>