[2831] Reset the parent's segment before forking

Though we won't have two readers at the same time, it's best we
reset the segment so that it's not carried over for the child
to use accidentally.

src/lib/util/tests/memory_segment_mapped_unittest.cc

@@ -384,6 +384,7 @@ TEST_F(MemorySegmentMappedTest, multiProcess) {
     ptr = segment_->getNamedAddress("test address");
     ASSERT_TRUE(ptr);
     EXPECT_EQ(424242, *static_cast<const uint32_t*>(ptr));
+    segment_.reset();
 
     // Spawn another process and have it open and read the same data
     int pipes[2];