imfs: Fix index underrun when extending empty file

Currently the following sequence causes a endless loop when extending an
IMFS file:

- Create a file with zero length and close it.
- Make sure nearly no allocatable memory is left.
- Open the file and write enough data into it that more than the
  remaining memory will be used.

In that case when extending the IMFS file, the file currently need zero
blocks. If allocating enough new blocks fails, the already allocated new
blocks will be freed again.

The comparison of block>=old_blocks that has been used prior to this
patch compared two unsigned numbers. If old_blocks was zero, the
comparison of these two numbers always evaluated to true.

This patch frees the last block in a separate step to avoid this
problem.

Fixes #4638

0 files changed, 0 insertions, 0 deletions