Problem was that in mysql-trunk the ER() macro is now dependent on current_thd
and the innodb monitor thread has no binding to that thd object. This cause 
the crash because of bad derefencing.

Solution was to add a new macro which take the thd as an argument (which the innodb
thread uses for the call).

(Updated according to reviewers comments, i.e. added ER_THD_OR_DEFAULT and
moved test to suite parts.)

mysql-test/suite/parts/r/partition_innodb_status_file.result:
  Bug#50201: Server crashes in explain_filename on an InnoDB partitioned table
  
  New test result file
mysql-test/suite/parts/t/partition_innodb_status_file-master.opt:
  Bug#50201: Server crashes in explain_filename on an InnoDB partitioned table
  
  New test opt file
mysql-test/suite/parts/t/partition_innodb_status_file.test:
  Bug#50201: Server crashes in explain_filename on an InnoDB partitioned table
  
  New test.
  Note that the innodb monitor thread only runs every 15 seconds, so this
  test will take at least 15 seconds, so I have moved it to the parts suite.
sql/sql_table.cc:
  Bug#50201: Server crashes in explain_filename on an InnoDB partitioned table
  
  Using thd safe ER macro.
sql/unireg.h:
  Bug#50201: Server crashes in explain_filename on an InnoDB partitioned table
  
  Added ER macros for use with specified thd pointer.