buffer: add an assert for last_with_datap to suppress static analyzer

buffer: add an assert for last_with_datap to suppress static analyzer

  ../buffer.c:2231:6: warning: Access to field 'flags' results in a dereference of a null pointer
          if (CHAIN_SPACE_LEN(*firstchainp) == 0) {
              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../buffer.c:130:30: note: expanded from macro 'CHAIN_SPACE_LEN'
  #define CHAIN_SPACE_LEN(ch) ((ch)->flags & EVBUFFER_IMMUTABLE ? \