[lldb] Remove lldbassert from CommandInterpreter::PrintCommandOutput

The assertion checks that the command output doesn't contain any null
bytes. I'm not sure if the intention was to make sure the string wasn't
shorter than the reported length or if this was a way to catch us
accidentally writing an (unformatted) null byte.

The consensus is that we don't want to have embedded nulls in the
command output, but that this isn't the right place to enforce that.

Differential revision: https://reviews.llvm.org/D122025