Assert acount hash mmap file capacity > 0

[HaoranYi]

Problem

We are seeing a memory allocation failure when writing 0 to the end of
account's hash file.

It seems that it could be caused by self.capacity = 0. When we estimate the
capacity, it is possible that self.capacity = 0.

However, if write() is called, self.capacity must be greater than 0.

Summary of Changes

To rule out the possiblity, add an "assert" for capacity must be greater than
0.

Fixes #

[jeffwashington]

lgtm

[codecov]

Codecov Report

Merging #33575 (a9ac7c8) into master (7afb11f) will decrease coverage by 0.1%.
Report is 3 commits behind head on master.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##           master   #33575     +/-   ##
=========================================
- Coverage    81.7%    81.7%   -0.1%     
=========================================
  Files         807      807             
  Lines      218287   218288      +1     
=========================================
- Hits       178524   178482     -42     
- Misses      39763    39806     +43