When a mismatching end macro occurs while at least two nested blocks

When a mismatching end macro occurs while at least two nested blocks
are open, all except the innermost open block got a bogus MDOC_ENDED
marker, in some situations triggering segfaults down the road
which tb@ found with afl(1).
Fix the logic error by figuring out up front whether an end macro
has a matching body, and if it hasn't, don't mark any blocks as broken.