varlink: use two local flag variables to silence gcc warning

varlink: use two local flag variables to silence gcc warning

[59/655] Compiling C object src/shared/libsystemd-shared-248.a.p/varlink.c.o
../src/shared/varlink.c: In function ‘varlink_write’:
../src/shared/varlink.c:459:12: warning: ‘n’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  459 |         if (n < 0) {
      |            ^
../src/shared/varlink.c: In function ‘varlink_process’:
../src/shared/varlink.c:541:12: warning: ‘n’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  541 |         if (n < 0) {
      |            ^
../src/shared/varlink.c:486:17: note: ‘n’ was declared here
  486 |         ssize_t n;
      |                 ^