Get rid of compiler warning about `image->filename` (#500)
Get rid of compiler warning about `image->filename` (#500)

This patch will use expected type for variable to get rid of following compiler warning.

```
rmimage.c:8309:16: warning: address of array 'image->filename' will always evaluate to 'true' [-Wpointer-bool-conversion]
    if (image->filename)
    ~~  ~~~~~~~^~~~~~~~
1 warning generated.
```

`image->filename` has declared as following in ImageMagick.

```
  char
    magick[MaxTextExtent],
    unique[MaxTextExtent],
    zero[MaxTextExtent],
    filename[MaxTextExtent];
```

The address of `filename[MaxTextExtent]` always is not `NULL`.

So, this patch will remove unnecessary `if` sentence to get rid of compiler warning.
rmagick/rmagick,rmagick/rmagick,rmagick/rmagick