Fix: GPv3: Uninitialized runtime variable

The `eval_frame` was not initialized to a default value,
causing a crash in some situations on windows.
This at least avoids a crash even if the outcome of
whatever uses the `eval_frame` might not be correct.

source/blender/blenkernel/BKE_grease_pencil.hh

@@ -683,7 +683,7 @@ class GreasePencilRuntime {
    */
   void *batch_cache = nullptr;
   /* The frame on which the object was evaluated (only valid for evaluated object). */
-  int eval_frame;
+  int eval_frame = 0;
 
  public:
   GreasePencilRuntime() {}