ARROW-12906: [C++][Python] Fix fill_null segfault

Closes #10434 from cyb70289/12906-fill-null-crash

Authored-by: Yibo Cai <yibo.cai@arm.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/compute/kernels/scalar_fill_null.cc

@@ -162,7 +162,6 @@ struct FillNullFunctor<Type, enable_if_t<is_base_binary_type<Type>::value>> {
     const ArrayData& input = *batch[0].array();
     const auto& fill_value_scalar =
         checked_cast<const BaseBinaryScalar&>(*batch[1].scalar());
-    util::string_view fill_value(*fill_value_scalar.value);
     ArrayData* output = out->mutable_array();
 
     // Ensure the kernel is configured properly to have no validity bitmap /
@@ -172,6 +171,7 @@ struct FillNullFunctor<Type, enable_if_t<is_base_binary_type<Type>::value>> {
     const int64_t null_count = input.GetNullCount();
 
     if (null_count > 0 && fill_value_scalar.is_valid) {
+      util::string_view fill_value(*fill_value_scalar.value);
       BuilderType builder(input.type, ctx->memory_pool());
       RETURN_NOT_OK(builder.ReserveData(input.buffers[2]->size() +
                                         fill_value.length() * null_count));

python/pyarrow/tests/test_compute.py

@@ -1078,6 +1078,11 @@ def test_fill_null():
     expected = pa.array([b'a', b'bb', b'ccc'], type=pa.large_binary())
     assert result.equals(expected)
 
+    arr = pa.array(['a', 'bb', None])
+    result = arr.fill_null(None)
+    expected = pa.array(['a', 'bb', None])
+    assert result.equals(expected)
+
 
 @pytest.mark.parametrize('arrow_type', numerical_arrow_types)
 def test_fill_null_array(arrow_type):