gh-36445: fix ZeroDivisionError in gcd of sparse polynomials
Fixes #36427
As pointed out in #36427, attempting to calculate the gcd of two sparse
polynomials raises a `ZeroDivisionError` if one of the polynomials is
constant, and the other has large degree (at least 100).
This is caused by an error in the code that tests whether converting
either of the polynomials to FLINT will be very slow. To fix this, we
check each polynomial individually, instead of trying to consider both
polynomials at once.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
URL: https://github.com/sagemath/sage/pull/36445
Reported by: DaveWitteMorris
Reviewer(s): John H. Palmieri
