Add missing bounds check in is_positive_entry(::ZZMatrix, ...)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.96%. Comparing base (784c28a) to head (b236df7).

Files	Patch %	Lines
src/flint/fmpz_mat.jl	0.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1802 +/- ## ========================================== + Coverage 85.95% 85.96% +0.01% ========================================== Files 95 95 Lines 36417 36418 +1 ========================================== + Hits 31303 31308 +5 + Misses 5114 5110 -4 ```

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