fetchurl: fix resolvedUrl eval on URLs without mirror:// prefix

Without the change `resolvedUrl` fails on non-mirror:// packages as:

    $ nix eval --impure --expr 'with import ./. {}; AMB-plugins.src.resolvedUrl'
    error:
       ...
       error: expected a list but found null: null

After ther change the eval works as expected:

    $ nix eval --impure --expr 'with import ./. {}; AMB-plugins.src.resolvedUrl'
    "http://kokkinizita.linuxaudio.org/linuxaudio/downloads/AMB-plugins-0.8.1.tar.bz2"

pkgs/build-support/fetchurl/default.nix

@@ -203,7 +203,10 @@ let
         else
           throw "unknown mirror:// site ${mirrorName}";
     in
-    if mirrorName == null then url else "${lib.head mirrorList}${lib.elemAt mirrorSplit 1}";
+    if mirrorSplit == null || mirrorName == null then
+      url
+    else
+      "${lib.head mirrorList}${lib.elemAt mirrorSplit 1}";
 in
 
 assert