Fix reading banner from homebrew ROMs

Some homebrew ROMs do not have a banner, and use a null value to indicate this.
Do not attempt to read the banner when this is the case.

src/NDSCart.cpp

@@ -1602,7 +1602,15 @@ bool LoadROM(const u8* romdata, u32 romlen)
     memcpy(CartROM, romdata, romlen);
 
     memcpy(&Header, CartROM, sizeof(Header));
+
+    if (!Header.BannerOffset)
+    {
+        memset(&Banner, 0, sizeof(Banner));
+    }
+    else
+    {
         memcpy(&Banner, CartROM + Header.BannerOffset, sizeof(Banner));
+    }
 
     printf("Game code: %.4s\n", Header.GameCode);