Suppress compiler warning in new regex match-all detection code.
gcc 10 is smart enough to notice that control could reach this
"hasmatch[depth]" assignment with depth < 0, but not smart enough
to know that that would require a badly broken NFA graph. Change
the assert() to a plain runtime test to shut it up.
Per report from Andres Freund.
Discussion: https://postgr.es/m/
20210223173437.b3ywijygsy6q42gq@alap3.anarazel.de