MDEV-6512: InnoDB: Assertion failure in thread 4537024512 in file
buf0buf.cc line 2642.

Analysis: innodb_compression_algorithm is a global variable and
can change while we are building page compressed page. This could
lead page corruption.

Fix: Cache innodb_compression_algorithm on local variable before
doing any compression or page formating to avoid concurrent
change. Improved page verification on debug builds.