utils: lockfile: avoid stack overflow for lockfile buffer

There appears to have been some change on openSUSE (likely some new
hardening flags for builds, or some glibc hardening) such that incorrect
buffer handling results in a segfault even if the buffer is never
overflowed.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

src/utils/lockfile.cc

@@ -75,8 +75,9 @@ Lockfile::try_lock() {
   int  pos = ::gethostname(buf, 255);
 
   if (pos == 0) {
-    ::snprintf(buf + std::strlen(buf), 255, ":+%i\n", ::getpid());
-    ssize_t __attribute__((unused)) result = ::write(fd, buf, std::strlen(buf));
+    ssize_t len = std::strlen(buf);
+    ::snprintf(buf + len, 255 - len, ":+%i\n", ::getpid());
+    int __attribute__((unused)) result = ::write(fd, buf, std::strlen(buf));
   }
 
   ::close(fd);