Handle files that begin with comments in PrivateNamingConvention

After trying out this new rule, I noticed a couple of bugs. Both are
related, and one caused a crash. They both happen when the file begins
with a comment.

For some reason, if the file starts with a comment, the engine seems to
visit the function usage twice, once before it visits the function
definition, and once after. Since we are relying on the order that these
things are visited here, this causes a lint to happen.

But even worse, when visiting a variable within a for loop, the node has
no source range, which causes a crash. This only seems to happen when
the file begins with a comment.

I fixed these by making sure that things have a source range before we
determine their position, and by also checking the top level even though
we really shouldn't need to.

lib/scss_lint/linter/private_naming_convention.rb

@@ -105,10 +105,7 @@ def node_defined_earlier_in_branch?(node_to_look_in, looking_for)
         return true # We found a match, so we are done
       end
 
-      # We are at the top of the branch and don't want to check the root branch,
-      # since that is handled elsewhere, which means that we did not find a
-      # match.
-      return false unless node_to_look_in.node_parent.node_parent
+      return false unless node_to_look_in.node_parent
 
       # We did not find a match yet, and haven't reached the top of the branch,
       # so recurse.
@@ -122,6 +119,7 @@ def private?(node)
     end
 
     def before?(node, before_location)
+      return true unless node.source_range
       location = location_from_range(node.source_range)
       return true if location.line < before_location.line
       if location.line == before_location.line &&

spec/scss_lint/linter/private_naming_convention_spec.rb

@@ -60,6 +60,38 @@
       it { should_not report_lint }
     end
 
+    context 'is defined and used in a for loop when the file begins with a comment' do
+      let(:scss) { <<-SCSS }
+        // A comment
+        @function _foo() {
+          @return red;
+        }
+
+        @for $i from 0 through 1 {
+          .foo {
+            color: _foo();
+          }
+        }
+      SCSS
+
+      it { should_not report_lint }
+    end
+
+    context 'is defined and used in the same file when the file begins with a comment' do
+      let(:scss) { <<-SCSS }
+        // A comment
+        @function _foo() {
+          @return red;
+        }
+
+        .foo {
+          color: _foo();
+        }
+      SCSS
+
+      it { should_not report_lint }
+    end
+
     context 'is defined within a selector and not used' do
       let(:scss) { <<-SCSS }
         p {