QuadraticCost Does Not Check Matrix Shape about drake HOT 2 CLOSED

The size is checked in the debug mode

@jwnimmer-tri I can activate the check in the release mode. But what would be our general strategy? It is quite hard for python users to switch to debug mode, so shall we prefer DRAKE_DEMAND to DRAKE_ASSERT in our C++ code?

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See #7827. The functions DRAKE_DEMAND and DRAKE_ASSERT are only appropriate for Drake checking its own internal invariants. User input should always be checked with DRAKE_THROW_UNLESS (or something similar, like an if-test with a throw statement).

Ideally, any patches to this function would also change it obey https://drake.mit.edu/styleguide/cppguide.html#Inline_Functions, but if this an emergency maybe the platform reviewer will let you get away without doing that part.

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