perf tools: Do not remove addr_location.thread in thread__find_map()

perf tools: Do not remove addr_location.thread in thread__find_map()

The thread__find_map() is to find a map for a given address in the
given thread's address space.  It searches maps based on the cpu mode
and fills various information in the addr_location data structure.

It might change al->maps and al->map, but not al->thread.  Then I
think no reason to not set the al->thread at the beginning.

Also get rid of the duplicate 'al->map = NULL' part.

Fixes: 0dd5041c9a0ea ("perf addr_location: Add init/exit/copy functions")
Acked-by: Ian Rogers <irogers@google.com>
Acked-by: James Clark <james.clark@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>