Improve function call return value handling

Fix segfault caused by static cast and wrong type
Fixes #1269

Author: xzyfer

bind.cpp

@@ -142,9 +142,7 @@ namespace Sass {
           break;
         }
         // otherwise move one of the rest args into the param, converting to argument if necessary
-        if (arglist->is_arglist()) {
-          a = static_cast<Argument*>((*arglist)[0]);
-        } else {
+        if (!(a = dynamic_cast<Argument*>((*arglist)[0]))) {
           Expression* a_to_convert = (*arglist)[0];
           a = new (ctx.mem) Argument(a_to_convert->pstate(),
                                      a_to_convert,

eval.cpp

@@ -703,10 +703,8 @@ namespace Sass {
     if (result->pstate().file == string::npos)
       result->pstate(c->pstate());
 
-    do {
-      result->is_delayed(result->concrete_type() == Expression::STRING);
-      result = result->perform(this);
-    } while (result->concrete_type() == Expression::NONE);
+    result->is_delayed(result->concrete_type() == Expression::STRING);
+    if (!result->is_delayed()) result = result->perform(this);
     return result;
   }