RFC: fix Base.GMP.MPZ.export! on uninitialized vectors (#57787)

fixing Base.GMP.MPZ.export! not initializing memory when resizing vector
fixing Base.GMP.MPZ.export! not initializing memory, nonzero vectors are
only partially overwritten by :__gmpz_export, leading to unexpected
results
fixing Base.GMP.MPZ.export! not initializing memory when `n` is zero. if
`n` is zero :__gmpz_export does not touch target vector

adding test cases for fixed issues

---------

Co-authored-by: Rafael Fourquet <[email protected]>

Author: NegaScout

base/gmp.jl

@@ -257,6 +257,7 @@ function export!(a::AbstractVector{T}, n::BigInt; order::Integer=-1, nails::Inte
     stride(a, 1) == 1 || throw(ArgumentError("a must have stride 1"))
     ndigits = cld(sizeinbase(n, 2), 8*sizeof(T) - nails)
     length(a) < ndigits && resize!(a, ndigits)
+    fill!(a, zero(T))
     count = Ref{Csize_t}()
     ccall((:__gmpz_export, libgmp), Ptr{T}, (Ptr{T}, Ref{Csize_t}, Cint, Csize_t, Cint, Csize_t, mpz_t),
         a, count, order, sizeof(T), endian, nails, n)

test/gmp.jl

@@ -481,6 +481,18 @@ end
     bytes_to_export_to = Vector{UInt8}(undef, 2)
     Base.GMP.MPZ.export!(bytes_to_export_to, int_to_export_from, order=0)
     @test all(bytes_to_export_to .== bytes_to_import_from)
+
+    # test export of 0 is T[0]
+    zero_to_export = BigInt(0)
+    bytes_to_export_to = Vector{UInt8}(undef, 0)
+    Base.GMP.MPZ.export!(bytes_to_export_to, zero_to_export, order=0)
+    @test bytes_to_export_to == UInt8[0]
+
+    # test export on nonzero vector
+    x_to_export = BigInt(6)
+    bytes_to_export_to = UInt8[1, 2, 3, 4, 5]
+    Base.GMP.MPZ.export!(bytes_to_export_to, x_to_export, order=0)
+    @test bytes_to_export_to == UInt8[6, 0, 0, 0, 0]
 end
 
 @test isqrt(big(4)) == 2