[libstc++] Don't throw in std::assoc_legendre for m > l

[André Brand]

The return value specified in "8.1.2 associated Legendre polynomials"
of ISO/IEC JTC 1/SC 22/WG 21 N3060 (which is identical to the
expression in the doxygen comment of the patched function) is well-
defined for m>l: it is always zero because $ P_l(x) $ is a polynomial
of degree l.

The standard does not enforce an exception in this case because none of
the requirements in 8.1 (5) on page 11 of ISO/IEC JTC 1/SC 22/WG 21
N3060 are met.

Note: the implementation of st::assoc_legendre in Visual Studio 2017
(tested with Visual Studio 15.9.7) silently returns zero.
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