Fix T104039: GPencil Fill crash in multiframe

The problem was that the stroke array was not reset for 
each iteration of the loop, so the second time around, 
the array was not initialized correctly and 
was left with a NaN value.

source/blender/editors/gpencil/gpencil_fill.c

@@ -2919,6 +2919,7 @@ static int gpencil_fill_modal(bContext *C, wmOperator *op, const wmEvent *event)
               }
 
               if (extend_lines) {
+                stroke_array_free(tgpf);
                 gpencil_delete_temp_stroke_extension(tgpf, true);
               }