Remove null support in SharedFileRegion

ParcelFileDescriptor does not support null (negative) file descriptors
so we remove it from SharedFileRegion and expect that a @nullable
SharedFileRegion is used for cases where a null file descriptor is
allowed.

Test: atest shmemTest
Change-Id: Icf226181cf6de9759ed3145ff6b5ad0d8fd650af
diff --git a/media/libshmem/ShmemUtil.cpp b/media/libshmem/ShmemUtil.cpp
index a6d047f..e075346 100644
--- a/media/libshmem/ShmemUtil.cpp
+++ b/media/libshmem/ShmemUtil.cpp
@@ -19,6 +19,11 @@
 namespace media {
 
 bool validateSharedFileRegion(const SharedFileRegion& shmem) {
+    // FD must be valid.
+    if (shmem.fd.get() < 0) {
+        return false;
+    }
+
     // Size and offset must be non-negative.
     if (shmem.size < 0 || shmem.offset < 0) {
         return false;