Automatic strides are broken if one of the strides is UNK

[dsharlet]

Unknown strides are reassigned in resolve_unknowns:

array/array.h

Lines 347 to 348 in fba09f8

	if (dim0.stride() == UNK) {
	dim0.set_stride(current_stride);

which is incorrect if a stride is actually the value represented by UNK.

[Peter-McLean-Altera]

A candidate is std::dynamic_extent