Fix null pointer dereference in parse_selector_schema (#2372)

Spec sass/sass-spec#1107

Author: xzyfer

src/parser.cpp

@@ -558,12 +558,13 @@ namespace Sass {
           schema->append(str);
         }
 
-        // check if the interpolation only contains white-space (error out)
-        if (peek < sequence < optional_spaces, exactly<rbrace> > >(p+2)) { position = p+2;
-          css_error("Invalid CSS", " after ", ": expected expression (e.g. 1px, bold), was ");
-        }
         // skip over all nested inner interpolations up to our own delimiter
         const char* j = skip_over_scopes< exactly<hash_lbrace>, exactly<rbrace> >(p + 2, end_of_selector);
+        // check if the interpolation never ends of only contains white-space (error out)
+        if (!j || peek < sequence < optional_spaces, exactly<rbrace> > >(p+2)) {
+          position = p+2;
+          css_error("Invalid CSS", " after ", ": expected expression (e.g. 1px, bold), was ");
+        }
         // pass inner expression to the parser to resolve nested interpolations
         pstate.add(p, p+2);
         Expression_Obj interpolant = Parser::from_c_str(p+2, j, ctx, pstate).parse_list();