Fix maybe-uninitialized warnings with gcc 14.2

Fix maybe-uninitialized warnings with gcc 14.2

../src/resolve/resolved-bus.c: In function ‘call_link_method’:
../src/resolve/resolved-bus.c:1769:16: warning: ‘l’ may be used uninitialized [-Wmaybe-uninitialized]
 1769 |         return handler(message, l, error);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~
../src/resolve/resolved-bus.c:1755:15: note: ‘l’ was declared here
 1755 |         Link *l;
      |               ^
../src/resolve/resolved-bus.c: In function ‘bus_method_get_link’:
../src/resolve/resolved-bus.c:1828:13: warning: ‘l’ may be used uninitialized [-Wmaybe-uninitialized]
 1828 |         p = link_bus_path(l);
      |             ^~~~~~~~~~~~~~~~
../src/resolve/resolved-bus.c:1816:15: note: ‘l’ was declared here
 1816 |         Link *l;
      |               ^

(cherry picked from commit 5f911aca8434b4163514019fcb4c1c967a50617c)