Crash in OSM plugin with offline tiles if list of providers has changed or there is a file with special name

If using offline maps and providers list has changed, application crashes.

To reproduce:

• open minimal_map example
• add offline location to main.qml to point to some directory
• create a file in that directory that is named like:

  osm_100-l-8-10-510-512.png
  

Then start the example and it should come across an assert about index out of range or crash in release mode. This is because of number 8 in there, which is at the moment off by one.

Can also be reproduced with bigger provider number.