Fixed issue with corner case of reading empty fbcsr matrix

The issue was flagged by SonarCloud - we would be accessing invalid
memory in case the input was empty. This is now fixed.
Also added a test for the empty input case.

core/matrix/fbcsr.cpp

@@ -336,6 +336,11 @@ void Fbcsr<ValueType, IndexType>::read(const mat_data &data)
                              blocks.size() * bs * bs, bs);
 
     tmp->row_ptrs_.get_data()[0] = 0;
+    if (data.nonzeros.size() == 0) {
+        tmp->move_to(this);
+        return;
+    }
+
     index_type cur_brow = 0;
     index_type cur_bnz = 0;
     index_type cur_bcol = blocks.begin()->first.block_column;

core/test/matrix/fbcsr.cpp

@@ -460,6 +460,24 @@ TYPED_TEST(Fbcsr, CanBeReadFromMatrixData)
 }
 
 
+TYPED_TEST(Fbcsr, CanBeReadFromEmptyMatrixData)
+{
+    using Mtx = typename TestFixture::Mtx;
+    using MtxData = typename TestFixture::MtxData;
+    auto m = Mtx::create(this->exec);
+    m->set_block_size(this->fbsample.bs);
+    MtxData mdata;
+    mdata.size = gko::dim<2>{0, 0};
+
+    m->read(mdata);
+
+    ASSERT_EQ(m->get_size(), (gko::dim<2>{0, 0}));
+    ASSERT_EQ(m->get_const_row_ptrs()[0], 0);
+    ASSERT_EQ(m->get_const_col_idxs(), nullptr);
+    ASSERT_EQ(m->get_const_values(), nullptr);
+}
+
+
 TYPED_TEST(Fbcsr, GeneratesCorrectMatrixData)
 {
     using value_type = typename TestFixture::value_type;