~PathLocks(): Handle exceptions

Otherwise, since the call to write a "d" character to the lock file
can fail with ENOSPC, we can get an unhandled exception resulting in a
call to terminate().

src/libstore/pathlocks.cc

@@ -161,7 +161,11 @@ bool PathLocks::lockPaths(const PathSet & _paths,
 
 PathLocks::~PathLocks()
 {
-    unlock();
+    try {
+        unlock();
+    } catch (...) {
+        ignoreException();
+    }
 }