Add `eof` check

The addition of an eof check appears to cover a corner case detailed at https://github.com/BioJulia/XAM.jl/issues/31#issuecomment-787480072.

src/bam/overlap.jl

@@ -62,7 +62,7 @@ end
 function Base.iterate(iter::OverlapIterator, state)
     while state.chunkid ≤ lastindex(state.chunks)
         chunk = state.chunks[state.chunkid]
-        while BGZFStreams.virtualoffset(iter.reader.stream) < chunk.stop
+        while !eof(iter.reader) && BGZFStreams.virtualoffset(iter.reader.stream) < chunk.stop
             read!(iter.reader, state.record)
             c = compare_intervals(state.record, (state.refindex, iter.interval))
             if c == 0