basic/signal-util: drop unneeded extra byte from buffer

DECIMAL_STR_MAX() already returns +1 for NULL, so no need to do it
a second time.

src/basic/signal-util.c

@@ -198,7 +198,7 @@ static const char *const __signal_table[] = {
 DEFINE_PRIVATE_STRING_TABLE_LOOKUP(__signal, int);
 
 const char *signal_to_string(int signo) {
-        static thread_local char buf[STRLEN("RTMIN+") + DECIMAL_STR_MAX(int) + 1];
+        static thread_local char buf[STRLEN("RTMIN+") + DECIMAL_STR_MAX(int)];
         const char *name;
 
         name = __signal_to_string(signo);