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.