Changing the state of whether we're recording profiling information

halfway through a query (as happens in "SET SESSION PROFILING = ...")
has a few side-effects, the worst of which is a memory leak for
prepared statements, which poke directly from the parser into the 
profiling code (we don't have the query text when we need it) and 
that overwrites a pointer to heap-allocated memory when the previous
statement turns on profiling.

Instead, now set a flag when we begin a new statement that tracks 
whether profiling is on _at the start_ of the query.  Use that to
track whether we gather info.

Additionally, use that AND use the state of the profiling variable
after the end of a query to know whether to store information about 
the query that just finished.

sql/sql_profile.h

@@ -277,6 +277,7 @@ private:
   query_id_t profile_id_counter;     
   THD *thd;
   bool keeping;
+  bool enabled;
 
   QUERY_PROFILE *current;
   QUERY_PROFILE *last;