Introduced the new data member new_query_arena_is_set of the class sp_head.
This data member is used as a protection against double invocation of
the method restore_thd_mem_root that is called for restoration of the
current query arena. Previously, the data member sp_head::m_thd
is used for this goal but after support for recompilation
of failed stored routine statement has been added the data member
sp_head::m_thd can't be used for this goal. The reason is that
on statement recompilation after the method restore_thd_mem_root()
is called the method sp_head::add_instr() invoked to add a new instruction
for just recompiled statement. The method sp_head::add_instr()
de-references m_thd to access the free_list data member.
If m_thd was used as a guard against double invocation it would result in
a crash on dereferncing null pointer.