perf scripting: Avoid leaking the scripting_context variable

Both register_perl_scripting() and register_python_scripting() allocate
this variable, fix it by checking if it already was.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 7e4b21b8 ("perf/scripts: Add Python scripting engine")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>