When the fwd Gaussian elimination process encounters diagonal
element as zero value, the linear equation does not have unique
solution. Return the linear solver state as unsolvable in such
case. This resolves a floating point exception issue due to divided
by zero in the loop restoration filter.

BUG=aomedia:437

Change-Id: I3c67525691a3003f9f470e8a0d5b4ee187cba963