Patch for Bug#27863 (excessive memory usage for many small queries in a

multiquery packet).

Background:

  - a query can contain multiple SQL statements;

  - the server frees resources allocated to process a query when the
    whole query is handled. In other words, resources allocated to process
    one SQL statement from a multi-statement query are freed when all SQL
    statements are handled.

The problem was that the parser allocated a buffer of size of the whole
query for each SQL statement in a multi-statement query. Thus, if a query
had many SQL-statements (so, the query was long), but each SQL statement
was short, ther parser tried to allocate huge amount of memory (number of
small SQL statements * length of the whole query).

The memory was allocated for a so-called "cpp buffer", which is intended to
store pre-processed SQL statement -- SQL text without version specific
comments.

The fix is to allocate memory for the "cpp buffer" once for all SQL
statements (once for a query).

sql/sql_signal.cc

@@ -75,10 +75,6 @@ const LEX_STRING Diag_statement_item_names[]=
   { C_STRING_WITH_LEN("TRANSACTION_ACTIVE") }
 };
 
-Set_signal_information::Set_signal_information()
-{
-  clear();
-}
 
 Set_signal_information::Set_signal_information(
   const Set_signal_information& set)