Extend fails if selector fails to unify

[glebm]

repro;

x {
 @extend #{&};
 @extend #{&};
}

output:

Error: "x" failed to @extend "x".
       The selector "x" was not found.

expected output: `` (empty, no error as per #2250)

refs: sass/libsass#2782

[nex3]

Note that this reproduces with just x {@extend x}. It should produce no output.