CONC-514 ma_net_write_buff() invokes memcpy() on null pointer
CONC-514 ma_net_write_buff() invokes memcpy() on null pointer

When the function ma_net_write_buff() is invoked with len=0,
it may call memcpy() on a null pointer, which is flagged in
WITH_UBSAN builds.

MariaDB/mariadb-connector-c,MariaDB/mariadb-connector-c,MariaDB/mariadb-connector-c