Fix LEN_POSOBJ_Handler for atomic positional objects

[fingolfin]

Also add a test for this.

[codecov]

Codecov Report

Merging #1414 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1414      +/-   ##
==========================================
+ Coverage   63.42%   63.43%   +<.01%     
==========================================
  Files        1011     1011              
  Lines      339073   339078       +5     
  Branches    13761    13752       -9     
==========================================
+ Hits       215068   215104      +36     
+ Misses     120961   120925      -36     
- Partials     3044     3049       +5

Impacted Files	Coverage Δ
src/objects.c	76.41% <100%> (+0.65%)	⬆️
src/objects.h	80% <0%> (-0.49%)	⬇️
src/hpc/thread.c	47.14% <0%> (-0.2%)	⬇️
src/listfunc.c	76.77% <0%> (-0.18%)	⬇️
src/funcs.c	63.12% <0%> (-0.15%)	⬇️
hpcgap/src/stats.c	73.16% <0%> (-0.14%)	⬇️
src/vec8bit.c	72.31% <0%> (+0.03%)	⬆️
src/intfuncs.c	90.98% <0%> (+0.11%)	⬆️
hpcgap/src/plist.c	82.94% <0%> (+0.16%)	⬆️
src/hpc/threadapi.c	32.51% <0%> (+0.19%)	⬆️
... and 4 more