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 ? \ (cherry picked from commit f83ac92da9fff789135d1e5b9050653cf7fdb517)