perf map: Allow map__next() to receive a NULL arg

perf map: Allow map__next() to receive a NULL arg

Just like free(), return NULL in that case, will simplify the
for_each_entry_safe() iterators.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-pbde2ucn49khnrebclys9pny@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>