Span bug when using std::fill_n in Release Mode

Created by: williamfgc

to reproduce with ctest on Ubuntu 18.04.2 LTS using gcc 7.4.0: ctest -VV -R Span

5/5 Test #197: BPWriteReadSpan.BPWriteRead1D8FillValue ...***Failed    0.35 sec

The following tests passed:
	BPWriteReadSpan.BPWriteRead1D8
	BPWriteReadSpan.BPWriteRead2D2x4
	BPWriteReadSpan.BPWriteRead1D8Local
	BPWriteReadSpan.BPWriteRead2D2x4Local

80% tests passed, 1 tests failed out of 5

Total Test time (real) =   1.19 sec

The following tests FAILED:
	197 - BPWriteReadSpan.BPWriteRead1D8FillValue (Failed)
Errors while running CTest