This patch borrows ideas, text and code from Kristofer
Pettersson's patch.

An assignment of a system variable sharing the same base
name as a declared stored procedure variable in the same
context could lead to a crash.

The reason was that during the parsing of the syntactic
rule 'option_value' an uninitialized set_var object was
pushed to the parameter stack of the SET statement. The
parent rule 'option_type_value' interpreted the existence
of variables on the parameter stack as an assignment and
wrapped it in a sp_instr_set object.

As the procedure later was executed an attempt was made
to run the method 'check()' on an uninitialized member
object (NULL value) belonging to the previously created
but uninitialized object.

This patch refactors the 'internal_variable_name' rule and
copies the semantic analysis part to the depending parent
rule: 'option_value'. This makes it possible to account
for any prefixes affecting the interpretation of the
internal_variable_name.

mysql-test/r/sp.result:
  Add test case result.
mysql-test/t/sp.test:
  Add test case for bug.
sql/sql_yacc.yy:
  - Reduce churn in rule sys_option_value by moving to
    specialized functions.
  - Comment the the lookup in the rule internel_variable_name
    is a best effort operation.
  - Lookup for a system variable in the option_value if one was
    not found (the variable could have been shadowed)