Problem:

It was reported an intermittent ASan error on rpl.rpl_checksum_cache.

The error informed that the binlog sender did a heap-use-after-free on
a given memory address.

Analysis:

The binlog sender read_event function will read an event from the
binary log an will return, among other things, a pointer to a buffer
containing the event.

The read_event function will pre-allocate the space needed to read an
event from the binary log to memory by calling reset_transmit_packet
function.

Then, it sets the pointer of the buffer that will contain the event to
be returned to the caller.

After setting the pointer, it calls Log_event::read_log_event function,
that might change the buffer size (replacing its pointer) if needed.

This is happening sometimes because of the need of memory alignment of
the buffer.

When read_log_event replaced the buffer, the previously pointer set
became invalid, and that is the error ASan is getting.

Fix:

Moved the event pointer attribution to a safer place.

Fixed a condition at grow_packet to avoid a packet to be resized when
calling read_log_event.

Fixed also an synchronization issue in the test case itself.