Fix segfault with calc prefix and underscore vs hyphen

Author: mgreter

src/eval.cpp

@@ -828,7 +828,7 @@ namespace Sass {
       Backtrace here(backtrace(), c->pstate(), ", in function `" + c->name() + "`");
       exp.backtrace_stack.push_back(&here);
       // eval the body if user-defined or special, invoke underlying CPP function if native
-      if (body && !Prelexer::re_special_fun(c->name().c_str())) { result = body->perform(this); }
+      if (body && !Prelexer::re_special_fun(name.c_str())) { result = body->perform(this); }
       else if (func) { result = func(fn_env, *env, ctx, def->signature(), c->pstate(), backtrace()); }
       if (!result) error(std::string("Function ") + c->name() + " did not return a value", c->pstate());
       exp.backtrace_stack.pop_back();