perf util: Fix memory leak of prefix_if_not_in

Need to free "str" before return when asprintf() failed to avoid memory
leak.

Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Hongbo Yao <yaohongbo@huawei.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Li Bin <huawei.libin@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20200521133218.30150-4-liwei391@huawei.com


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