Fix null pointer dereference in css_error (#2370)

Originally reported by @MrTuxracer via HackerOne.

Fixes #2369
Spec sass/sass-spec#1105

Author: xzyfer

src/parser.cpp

@@ -2869,6 +2869,7 @@ namespace Sass {
     const char* end = this->end;
     while (*end != 0) ++ end;
     const char* pos = peek < optional_spaces >();
+    if (!pos) pos = position;
 
     const char* last_pos(pos);
     if (last_pos > source) {