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

[Jonathan Wakely]

On 04/03/19 13:57 -0500, Ed Smith-Rowland wrote:
>This is actually PR libstdc++/86655.
>
>Thank you for reminding me Andre.
>
>I remove the throw for m > l and just return 0.  This is also done for 
>sph_legendre.
>
>This build and tests clean on x86_64-linux.
>
>OK?

OK for trunk, thanks.