mp.c: mp_reciprocal: fix when calling it with same arguments.

[mbkma]

The z value was set to 1, which is the same as the x value if both addresses x and z are equal. Thus, the final value of z was 1/1=1. Solved by making a temp variable.

No need to backport, because culprit b0117b1d5ae73916c6f0d289be1f693bb5f46824 is not in the 1.24 branch.

[raveit65]

How can this be tested? Before? After? :)

[mbkma]

For example this fixes currency conversion (was broken by b0117b1d5ae73916c6f0d289be1f693bb5f46824) because mp_reciprocal is used in currency manager:

$ grep -r mp_reciprocal
src/currency-manager.c:                    mp_reciprocal(&value, &value);

Test: Before: (most currencies are equally valuable) After: (most currencies are different)