Fix crash computing zero(ZZ[t]) - zero(ZZ)

The following lead to a crash (uncovered by Nemocas/AbstractAlgebra.jl#1867)
```julia
using Nemo
R, t = ZZ[:t]
zero(R) - ZZ(0)
```
This is due to a bug in FLINT's `fmpz_poly_sub_fmpz`.

src/flint/fmpz_poly.jl

@@ -875,7 +875,13 @@ function sub!(z::ZZPolyRingElemOrPtr, x::ZZPolyRingElemOrPtr, y::ZZPolyRingElemO
 end
 
 function sub!(z::ZZPolyRingElemOrPtr, x::ZZPolyRingElemOrPtr, y::ZZRingElemOrPtr)
-  @ccall libflint.fmpz_poly_sub_fmpz(z::Ref{ZZPolyRingElem}, x::Ref{ZZPolyRingElem}, y::Ref{ZZRingElem})::Nothing
+  if is_zero(y)
+    # HACK HACK HACK: workaround a crash in fmpz_poly_sub_fmpz when subtracting
+    # 0 from a zero polynomial; see https://github.com/flintlib/flint/pull/2102
+    set!(z, x)
+  else
+    @ccall libflint.fmpz_poly_sub_fmpz(z::Ref{ZZPolyRingElem}, x::Ref{ZZPolyRingElem}, y::Ref{ZZRingElem})::Nothing
+  end
   return z
 end
 

test/flint/fmpz_poly-test.jl

@@ -152,6 +152,9 @@ end
   @test 12 - g == -x^3-3*x+10
 
   @test ZZRingElem(12) - g == -x^3-3*x+10
+
+  # verify bugfix (used to crash)
+  @test is_zero(zero(R) - ZZ(0))
 end
 
 @testset "ZZPolyRingElem.comparison" begin