LinkSearch: use default protocol if unsupported

Default $protocol to http:// if part of link up to
colon or double-slash isn't in $wgUrlProtocols.

Also simplify elseif condition above - $protocol is
always blank there since neither if block above it
is executed.

Bug: 33029
Change-Id: I66d2c592998c750fe3da937b228fe1387fcd344c