Fix elusive bug in expairseq ctor.

Fix elusive bug in expairseq ctor.

When an expair turns out to represent a number, that should go into
the expairseq's overall_coeff. This was accomplished by class mul,
thanks to the override of expair_needs_further_processing(), but not
always for class add.

This patch fixes the base class' expair_needs_further_processing()
with similar logic as that already in place for class mul.

Thanks to Mario Prausa for reporting this.