Fix undefined behaviour in IntraPrediction::xTMPFusionApplyModel since AG0136

m_tmpFusionInfoUseMR and m_tmpFusionInfo are arrays with length 6. In the case cu->tmpFusionFlag is true, it is guaranteed that cu->tmpIdx is in [0..5] and all is good. If cu->tmpFusionFlag is false, however, cu->tmpIdx can take larger values and the array is accessed out-of-bounds which is undefined behaviour. Previously this was guaranteed by the short-circuiting of the comparison operators, but since AG0136 this is no longer the case and we need to take a bit more care here.

UBSAN report:

/Users/frank/ecm/source/Lib/CommonLib/IntraPrediction.cpp:21753:45: runtime error: index 7 out of bounds for type 'IntraTMPFusionInfo[6]'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Users/frank/ecm/source/Lib/CommonLib/IntraPrediction.cpp:21753:45