perf util: Fix perf_exe() buffer write past end

perf util: Fix perf_exe() buffer write past end

perf_exe() passes len to readlink() and then unconditionally writes a
trailing NUL at buf[n]. If readlink() returns len, the write lands one
byte past the buffer.

Read at most len - 1 bytes and keep the existing NUL termination. Also
guard the fallback path for tiny buffers so copying "perf" cannot
overflow.

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Miguel Martín Gil <miguel.martin.gil.uni@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>