sql: enhance vdbe_decode_msgpack_into_mem()

Currently, vdbe_decode_msgpack_into_mem() creates a MEM that is not
properly initialized in case msgpack contains MP_EXT, MP_MAP, or
MP_ARRAY fields. This patch makes it so that after execution of this
function, all created MEMs are properly initialized.

Closes #5011
Closes #5704
Closes #5913
Needed for #5818