Make VMString invalidation more general

Signed-off-by: Stefan Marr <[email protected]>

src/vmobjects/VMString.cpp

@@ -56,19 +56,11 @@ void VMString::MarkObjectAsInvalid() {
     for (size_t i = 0; i < length; i++) {
         chars[i] = 'z';
     }
+    chars = (char*) INVALID_GC_POINTER;
 }
 
 bool VMString::IsMarkedInvalid() const {
-    if (length == 0) {
-        return false;
-    }
-
-    for (size_t i = 0; i < length; i++) {
-        if (chars[i] != 'z') {
-            return false;
-        }
-    }
-    return true;
+    return chars == (char*) INVALID_GC_POINTER;
 }
 
 void VMString::WalkObjects(walk_heap_fn) {

src/vmobjects/VMString.h

@@ -62,7 +62,7 @@ class VMString: public AbstractVMObject {
     //this could be replaced by the CHARS macro in VMString.cpp
     //in order to decrease the object size
     const size_t length;
-    char* const chars;
+    char* chars;
 protected:
     VMString(char* const adaptedCharsPointer, const size_t length) :
         // set the chars-pointer to point at the position of the first character