qga: fix compiler warnings (clang 5)

static code analyzer complain:

qga/commands-posix.c:2127:9: warning: Null pointer passed as an argument to a 'nonnull' parameter
        closedir(dp);
        ^~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qga/commands-posix.c

@@ -2125,9 +2125,11 @@ static void transfer_memory_block(GuestMemoryBlock *mem_blk, bool sys2memblk,
          * we think this VM does not support online/offline memory block,
          * any other solution?
          */
-        if (!dp && errno == ENOENT) {
-            result->response =
-                GUEST_MEMORY_BLOCK_RESPONSE_TYPE_OPERATION_NOT_SUPPORTED;
+        if (!dp) {
+            if (errno == ENOENT) {
+                result->response =
+                    GUEST_MEMORY_BLOCK_RESPONSE_TYPE_OPERATION_NOT_SUPPORTED;
+            }
             goto out1;
         }
         closedir(dp);