libgmath: fix incorrect memset of double array

[nilason]

Fixes the following compiler (clang 12) warning:

la.c:93:34: warning: 'memset' call operates on objects of type 'doublereal' (aka 'double') while the size is based on a different type 'doublereal *' (aka 'double *') [-Wsizeof-pointer-memaccess]
    memset(A->vals, 0, sizeof(A->vals));
           ~~~~~~~            ~~~^~~~
la.c:93:34: note: did you mean to dereference the argument to 'sizeof' (and multiply it by the number of elements)?
    memset(A->vals, 0, sizeof(A->vals));
                              ~~~^~~~

[nilason]

Thanks @marisn !

Again, could someone backport this please.