Avoid holding reference to a GAP bag when NewBag is called.

[ChrisJefferson]

In some calls to CallErrorInner, one or more of the arguments
is a C pointer to a string stored in a GAP bag.
In that case we must call ErrorMessageToGAPString before NEW_PREC,
which could cause a GC which would move the string.

This can be observed using the memory checker in #2293 , type:

gap> GASMAN_MEM_CHECK(1);
gap> FASJKLFJSKLA; # Or any other invalid variable name

[codecov]

Codecov Report

Merging #2294 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2294      +/-   ##
==========================================
+ Coverage   71.77%   71.77%   +<.01%     
==========================================
  Files         479      479              
  Lines      251663   251663              
==========================================
+ Hits       180625   180632       +7     
+ Misses      71038    71031       -7

Impacted Files	Coverage Δ
src/gap.c	63.57% <100%> (+0.33%)	⬆️
src/hpc/threadapi.c	37.42% <0%> (ø)	⬆️
src/gasman.c	85.47% <0%> (+0.18%)	⬆️
src/objset.c	81.49% <0%> (+0.22%)	⬆️
hpcgap/lib/hpc/stdtasks.g	38.87% <0%> (+0.25%)	⬆️