display: Close 0 fds

0 can be a valid file descriptor, make sure 0 fds are closed

Change-Id: I2bf2280d70eda74fa0b0a886ea54ecefc6854b24
diff --git a/libexternal/external.cpp b/libexternal/external.cpp
index 1c8f170..883a39f 100644
--- a/libexternal/external.cpp
+++ b/libexternal/external.cpp
@@ -450,7 +450,7 @@
 bool ExternalDisplay::closeFrameBuffer()
 {
     int ret = 0;
-    if(mFd > 0) {
+    if(mFd >= 0) {
         ret = close(mFd);
         mFd = -1;
     }