fossil stash aborts when the changes are only deletions.

<verbatim>
$ fossil open ../test.fossil
...
t.txt
...
$ fossil rm t.txt
DELETED t.txt
$ fossil stash
Assertion failed: ((pBlob)->xRealloc==blobReallocMalloc || (pBlob)->xRealloc==blobReallocStatic), function blob_reset, file line 184.
Abort trap
</verbatim>