Fix out-of-bounds access on invalid mjvCamera->fixedcamid

[davidhozic]

The mjv_updateCamera method currently does bound checking for the fixedcamid only after calling both mjv_cameraFrame and mjv_cameraFrustum. Unlike mjv_cameraFrustum, the mjv_cameraFrame method does not do bound checking for the fixedcamid, which results in out-of-bounds access and thus undefined behavior, when fixedcamid is outside the valid range.

While the program will exit in error via mjERROR most of the time, this is still undefined behavior and can lead to random segmentation faults with no clear errors about what happened.

This pull request fixes this by reordering the method calls, resulting in the fixed camera's ID being checked prior to first indexing. A redundant check was also removed in mjv_updateCamera, because mjv_cameraFrustum already does the checking.