Fix Memory leaks

There wer 2 memory leaks reported by static analysis tool `scan-build` from Clang compiler. 
1. First was found in bt.c where return of slurp function was not freed, may be since the full functionality is not implemented yet. This is done in the second commit in this pull request.
2. Second was found in `benc_decode` function, I think there was typo in code and it was checking `strlen(in)` to be < 1 in place where it should be actually checking for allocation failure.