MP crashes when opening an MJPEG stream

#### Issue details
Mission Planner seems to crash almost immediately when opening an MJPEG stream. To reproduce just "Set MJPEG Source" and wait a few seconds.

I was able to reproduce this issue with the following public stream: https://webcam1.lpl.org/axis-cgi/mjpg/video.cgi

More streams for testing are available [here](https://en.ipcams.ch/WebCam.aspx?nr=1729).

#### Version
1.3.70

#### Platform
[ X ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

Start by reproducing the crash in Mission Planner using Set MJPEG Source with the linked public stream, then trace the failure in the MJPEG-stream opening path. Done means Mission Planner remains usable instead of crashing when opening the reported stream, with the reproduction scenario covered by an appropriate regression check.

Written by the indexing model from the issue text.