Fix clang warnings

Author: mgreter

src/expand.cpp

@@ -980,7 +980,7 @@ namespace Sass {
         */
 
     const char* cstr = text.c_str();
-    Sass_Import_Entry parent = ctx.import_stack.back();
+    // Sass_Import_Entry parent = ctx.import_stack.back();
     SelectorParser p2(ctx, cstr, "foobar", 0);
     if (block_stack.size() > 2) {
       Block* b = block_stack.at(block_stack.size() - 2);

src/parser.cpp

@@ -42,7 +42,7 @@ namespace Sass {
       auto resultasd = p2.identifier();
       return p2.scanner.isDone();
     }
-    catch (Exception::InvalidSyntax err) {
+    catch (Exception::InvalidSyntax& err) {
       return false;
     }
   }

src/parser_stylesheet.cpp

@@ -356,7 +356,7 @@ namespace Sass {
       }
 
     }
-    catch (Exception::InvalidSyntax err) {
+    catch (Exception::InvalidSyntax& err) {
       if (!couldBeSelector) throw;
 
       // If the value would be followed by a semicolon, it's
@@ -1039,7 +1039,7 @@ namespace Sass {
           return imp.detach();
         }
       }
-      catch (Exception::InvalidSyntax err) {
+      catch (Exception::InvalidSyntax& err) {
         // ToDo: refactor to return and throw here
         error("Invalid URL: ${innerError.message}",
           err.pstate
@@ -2723,7 +2723,7 @@ relase. For details, see http://bit.ly/moz-document.
         try {
           buffer.write(rawText(&StylesheetParser::_number));
         }
-        catch (Exception::InvalidSyntax err) {
+        catch (Exception::InvalidSyntax& err) {
            return false;
         }
         break;