Med: get_local_id: denoopize dependent condition (signedness issue)

Otherwise, condition in booth_tcp_init (src/transport.c):

	if (get_local_id() < 0)
		return -1;

could never be triggered.  It should not be triggered anyway at the
current state, but try to stay sane no matter what, especially if
the issue was hidden in such a nasty way.  Also note that using "int"
should be pretty much fine as precautions are taken in add_site
so as to keep MSB unset for site IDs in use, which is sufficient
to distinguish them from a signalling negative value.

Discovered-by: static analysis/Coverity

src/inline-fn.h

@@ -29,7 +29,7 @@
 
 
 
-inline static uint32_t get_local_id(void)
+inline static int get_local_id(void)
 {
 	return local ? local->site_id : -1;
 }