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)
          |            ^

src/basic/tmpfile-util.c

@@ -67,7 +67,7 @@ int fopen_temporary(const char *path, FILE **ret_f, char **ret_temp_path) {
 
 /* This is much like mkostemp() but is subject to umask(). */
 int mkostemp_safe(char *pattern) {
-        int fd;
+        int fd = -1; /* avoid false maybe-uninitialized warning */
 
         assert(pattern);