We now get an run-time error instead of a crash (although a slightly misleading
error message, but it's an improvement).


mysql-test/r/sp-error.result:
  New test case for BUG#2653.
mysql-test/t/sp-error.test:
  New test case for BUG#2653.
sql/sp_head.cc:
  Detect failed evals (fix item really), which are due to unresolved variables/fields.
  Typically this would be a reference to an undeclared variable.
  (Also got rid of some compiler warnings.)
sql/sp_rcontext.cc:
  Detect failed evals (fix item really), which are due to unresolved variables/fields.
  Typically this would be a reference to an undeclared variable.
sql/sp_rcontext.h:
  Changed return type to int, so be able to detect failed evals (fix item).
sql/sql_class.cc:
  Changed return type to int, so be able to detect failed evals (fix item).