Fix for BUG#13549 "Server crash with nested stored procedures

if inner routine has more local variables than outer one, and
one of its last variables was used as argument to NOT operator".

THD::spcont was non-0 when we were parsing stored routine/trigger
definition during execution of another stored routine. This confused
methods of Item_splocal and forced them use wrong runtime context.
Fix ensures that we always have THD::spcont equal to zero during
routine/trigger body parsing. This also allows to avoid problems
with errors which occur during parsing and SQL exception handlers.

sql/sql_class.h

@@ -2075,6 +2075,13 @@ public:
 class my_var : public Sql_alloc  {
 public:
   LEX_STRING s;
+#ifndef DEBUG_OFF
+  /*
+    Routine to which this Item_splocal belongs. Used for checking if correct
+    runtime context is used for variable handling.
+  */
+  sp_head *owner;
+#endif
   bool local;
   uint offset;
   enum_field_types type;