bugs in mpn_mul function

[游俊德]

Version: 4.1.3
File: mpn/generic/mul.c
Function: mpn_mul

The last line of mpn_mul function is not correct  ->  return prodp[un + vn -
l];
un + vn - l is not the correct index of most significant limb of prodp
array.

You can multiply two number by mpn_mul and mpn_mul_basecase separately
and compare the return value of these two functions which are different.