Fix Analyzer.debug_info_line to return nil on with-fallthrough

When :beam_lib.chunks fails (e.g. a project module hasn't been loaded
into :code.which yet), the existing with returned the raw error tuple
as the 'line' value, which then crashed format/1 via String.Chars.

Add an explicit else clause so the function always returns nil or an
integer.

lib/mix_unused/analyzer.ex

@@ -183,6 +183,8 @@ defmodule MixUnused.Analyzer do
         {{^name, ^arity}, _kind, meta, _clauses} -> Keyword.get(meta, :line)
         _ -> nil
       end)
+    else
+      _ -> nil
     end
   rescue
     _ -> nil