Fix typos in contracts/eosiolib/types.h documentation

#4972 created a different calling path into `producer_plugin_impl::calculate_next_block_time` where there was no pending block state.  As a result when the following conditions were met the code would dereference a null ptr:

* the node was a producer, who _had_ produced during this processes lifetime OR replayed a block where they produced (so they have watermark data)
* still in the active schedule of producers
* experienced a gap in blocks of > 5 seconds (so they moved to our “waiting” mode)

Credit goes to @heifner for the find

…roducer

Fix for nullptr deref when producers are waiting

Remove unused variable

…o/eos into gh#5281-required-keys-delay