Fix ScopedExprDeleter
Fix ScopedExprDeleter

When move assigning/constructing std::function the function moved from
is left in a valid but unspecified state. Care needs to be taken to make
sure ScopedExprDeleter accounts for that.

This commit fixes lifetime issues I observed in bpftrace compiled with
Android's r23 ndk.

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace