perf bpf: Avoid memory leak from perf_env__insert_btf()

In the Linux kernel, the following vulnerability has been resolved:

perf bpf: Avoid memory leak from perf_env__insert_btf()

perf_env__insert_btf() doesn't insert if a duplicate BTF id is encountered and this causes a memory leak. Modify the function to return a success/error value and then free the memory if insertion didn't happen.

v2. Adds a return -1 when the insertion error occurs in perf_env__fetch_btf. This doesn't affect anything as the result is never checked.