Clang UBSAN currently complains that the char * to input_buffer_adapter
is a nullptr.
Turns out it is actually required to accept nullptr, see for example
line 415 in input_adapters.hpp
...
// the address of first cannot be used: use nullptr
ia = std::make_shared<input_buffer_adapter>(nullptr, len);
....
Therefore we have to handle it gracefully here. We now also ignore the
length parameter l if b is a nullptr.
