Fix stack smashing error during gdb_mpq_write_fixed_point selftest

Fix stack smashing error during gdb_mpq_write_fixed_point selftest

When building GDB using Ubuntu 20.04's system libgmp and compiler,
running the "maintenance selftest" command triggers the following error:

    | Running selftest gdb_mpq_write_fixed_point.
    | *** stack smashing detected ***: terminated
    | [1]    1092790 abort (core dumped)  ./gdb gdb

This happens while trying to construct an mpq_t object (a rational)
from two integers representing the numerator and denominator.
In our test, the numerator is -8, and the denominator is 1.
The problem was that the rational was constructed using the wrong
function. This is what we were doing prior to this patch:

    mpq_set_ui (v.val, numerator, denominator);

The 'u' in "ui" stands for *unsigned*, which is wrong because
numerator and denominator's type is "int".

As a result of the above, instead of getting a rational value of -8,
we get a rational with a very large positive value (gmp_printf
says "18446744073709551608").

From there, the test performs an operation which is expected to
write this value into a buffer which was not dimensioned to fit
such a number, thus leading GMP into a buffer overflow.
This was verified by applying the formula that GMP's documentation
gives for the required memory buffer size needed during export:

    | When an application is allocating space itself the required size can
    | be determined with a calculation like the following. Since
    | mpz_sizeinbase always returns at least 1, count here will be at
    | least one, which avoids any portability problems with malloc(0),
    | though if z is zero no space at all is actually needed (or written).
    |
    |     numb = 8*size - nail;
    |     count = (mpz_sizeinbase (z, 2) + numb-1) / numb;
    |     p = malloc (count * size);

With the very large number, mpz_sizeinbase returns 66 and thus
the malloc size becomes 16 bytes instead of the 8 we allocated.

This patch fixes the issue by using the correct "set" function.

gdb/ChangeLog:

        * unittests/gmp-utils-selftests.c (write_fp_test): Use mpq_set_si
        instead of mpq_set_ui to initialize our GMP rational.