basic/tmpfile: avoid maybe-uninitialized warning in mkostemp_safe()

basic/tmpfile: avoid maybe-uninitialized warning in mkostemp_safe()

The variable is always initialized, but the compiler might not notice
that. With gcc-9.2.1-1.fc31:

    $ CFLAGS='-Werror=maybe-uninitialized -Og' meson build
    $ ninja -C build
    [...]
    ../src/basic/tmpfile-util.c: In function ‘mkostemp_safe’:
    ../src/basic/tmpfile-util.c:76:12: error: ‘fd’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
       76 |         if (fd < 0)
          |            ^