Added pointer checks to Android's resource stream.

This fixes issues #1056.

src/SFML/System/Android/ResourceStream.cpp

@@ -48,37 +48,68 @@ m_file (NULL)
 
 ////////////////////////////////////////////////////////////
 ResourceStream::~ResourceStream()
+{
+    if (m_file)
     {
         AAsset_close(m_file);
     }
+}
 
 
 ////////////////////////////////////////////////////////////
 Int64 ResourceStream::read(void *data, Int64 size)
+{
+    if (m_file)
     {
         return AAsset_read(m_file, data, size);
     }
+    else
+    {
+        return -1;
+    }
+}
 
 
 ////////////////////////////////////////////////////////////
 Int64 ResourceStream::seek(Int64 position)
+{
+    if (m_file)
     {
         return AAsset_seek(m_file, position, SEEK_SET);
     }
+    else
+    {
+        return -1;
+    }
+}
 
 
 ////////////////////////////////////////////////////////////
 Int64 ResourceStream::tell()
+{
+    if (m_file)
     {
         return getSize() - AAsset_getRemainingLength(m_file);
     }
+    else
+    {
+        return -1;
+    }
+}
 
 
 ////////////////////////////////////////////////////////////
 Int64 ResourceStream::getSize()
+{
+    if (m_file)
     {
         return AAsset_getLength(m_file);
     }
+    else
+    {
+        return -1;
+    }
+}
 
 
 } // namespace priv