fix out of bounds static_vector access in MIP
Fixes an assertion in debug mode.
In general the mode decision code should be refactored to use uiRdModeList.size() instead of numModesForFullRD, because these tend to diverge and cause issues. It's even mentioned in a comment above that numModesForFullRD should not be used beyond this point, but there are multiple locations (also outside MIP) using it.