Have to init. all local variables in their frames, not just once at the beginning
  of invocation.


mysql-test/r/sp.result:
  New test case for BUG#13133.
mysql-test/t/sp.test:
  New test case for BUG#13133.
sql/sp_head.cc:
  Just init. local variable slots in the fram to NULL. (Real init. will be done
  in each block.)
sql/sp_pcontext.cc:
  Removed isset flag, since it's not used.
sql/sp_pcontext.h:
  Removed isset flag, since it's not used.
sql/sql_yacc.yy:
  Initialize local variables in the block to null, or the default value, given.
  (Untabifed block too.)