The problem was that a syntactically invalid trigger could cause
the server to crash when trying to list triggers. The crash would
happen due to a mishap in the backup/restore procedure that should
protect parser items which are not associated with the trigger. The
backup/restore is used to isolate the parse tree (and context) of
a statement from the load (and parsing) of a trigger. In this case,
a error during the parsing of a trigger could cause the improper
backup/restore sequence.

The solution is to properly restore the original statement context
before the parser is exited due to syntax errors in the trigger body.

mysql-test/r/trigger.result:
  Add test case result for Bug#50755
mysql-test/t/trigger.test:
  Add test case for Bug#50755
sql/sp_head.cc:
  Merge sp_head::destroy() and sp_head destructor. Retrieve THD
  from the LEX so that m_thd is not necessary.
sql/sql_lex.cc:
  Explicitly restore the original environment.