Stop degree(::MPoly,...) from overflowing

[thofma]

Fixes the following feature:

julia> Qx, x = QQ["x1", "x2"]
(Multivariate polynomial ring in 2 variables over QQ, QQMPolyRingElem[x1, x2])

julia> degree(x[1]^(ZZ(2)^70), 1)
0

CC: @fieker

[thofma]

Edit: The check is a bit awkward, since one cannot check an individual variable for overflow.

[codecov]

Codecov Report

Merging #1524 (4c439b1) into master (38fd074) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1524      +/-   ##
==========================================
- Coverage   82.75%   82.75%   -0.01%     
==========================================
  Files          95       95              
  Lines       37186    37215      +29     
==========================================
+ Hits        30773    30796      +23     
- Misses       6413     6419       +6     

Files Changed	Coverage Δ
src/flint/fmpq_mpoly.jl	86.51% <100.00%> (+0.14%)	⬆️
src/flint/fmpz_mod_mpoly.jl	95.42% <100.00%> (+0.06%)	⬆️
src/flint/fmpz_mpoly.jl	86.53% <100.00%> (+0.15%)	⬆️
src/flint/fq_nmod_mpoly.jl	90.76% <100.00%> (+0.11%)	⬆️
src/flint/nmod_mpoly.jl	97.90% <100.00%> (+0.02%)	⬆️

... and 3 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more