CachedAppOptimizer: fix fd leak.

errno isn't modified when a syscall returns successfully: this would
leak if errno was already non-zero before entering this function.

Bug: http://b/172518739
Test: none
Change-Id: I38050863ceca226a9c2d143c85e0713c6a6fe511

services/core/jni/com_android_server_am_CachedAppOptimizer.cpp

@@ -86,7 +86,7 @@ static int compactMemory(const std::vector<Vma>& vmas, int pid, int madviseType)
 
     int pidfd = syscall(__NR_pidfd_open, pid, 0);
     err = -errno;
-    if (err < 0) {
+    if (pidfd < 0) {
         // Skip compaction if failed to open pidfd with any error
         return err;
     }