and base tables 

myrg_attach_children() could reuse a buffer that was allocated 
previously based on a definition of a child table. The problem 
was that the child's definition might have been changed, so 
reusing the buffer could lead to crashes or valgrind errors 
under some circumstances. 
 
Fixed by changing myrg_attach_children() so that the 
rec_per_key_part buffer is reused only when the child table
have not changed, and reallocated otherwise (the old buffer is 
deallocated if necessary).


include/myisammrg.h:
  Added a pointer to need_compat_check as an argument to
  myrg_attach_children().
mysql-test/r/merge.result:
  Added a test case for bug #45796.
mysql-test/t/merge.test:
  Added a test case for bug #45796.
storage/myisammrg/ha_myisammrg.cc:
  Pass a pointer to need_compat_check to myrg_attach_children().
storage/myisammrg/myrg_open.c:
  Changed myrg_attach_children() so that the 
  rec_per_key_part buffer is reused only when the child table
  have not changed, and reallocated otherwise (the old buffer 
  is deallocated if necessary).