defs >= &4000 at start results in no output

[stefandrissen]

org 0
dump 1,0
defs &3fff
defb 1

pyz80 --obj=space source.asm - results in the expected 16k object file.

org 0
dump 1,0
defs &4000
defb 1

pyz80 --obj=space source.asm - results in no object file at all.

[stefandrissen]

The use case for the above was to insert 16k to mdat "samdos" at 16384 because I was unable to get dZ80 to use an offset.

But I've found that the issue occurs when nothing has happened in a page yet. I'll create a PR.

[simonowen]

Fix merged, thanks!