Appropriate zero in _lu_tridiag! (#1374)

Working on #1372, I came across some unusual tests with non-Number
`eltype`s, for which this `fill!` fails unless the appropriate zero is
used. Probably missing convert methods, but we might as well use the
zero of the eltype here.

Author: jishnub

src/lu.jl

@@ -652,7 +652,7 @@ end
 
     # Initialize variables
     info = 0
-    fill!(du2, 0)
+    fill!(du2, zero(eltype(du2)))
 
     @inbounds begin
         for i = 1:n