Crash when multiplying matrices with undefined elements

[t-paul]

(reported on IRC)

Example:
echo([ [ 1, 2 ], undef ] * [ [ 4 ], [ 5 ] ]);

This triggers:

openscad: src/value.cc:464: static Value Value::multvecmat(const Value&, const Value&): Assertion `vectorvec.size() == matrixvec.size()' failed.