Fixed crash when using double-buffering with DGA

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%40203

src/video/dga/SDL_dgavideo.c

@@ -531,7 +531,6 @@ SDL_Surface *DGA_SetVideoMode(_THIS, SDL_Surface *current,
 			flip_address[1] = memory_base+screen_len;
 			surfaces_mem += screen_len;
 			surfaces_len -= screen_len;
-			DGA_FlipHWSurface(this, current);
 		}
 	}
 
@@ -542,6 +541,13 @@ SDL_Surface *DGA_SetVideoMode(_THIS, SDL_Surface *current,
 	}
 	DGA_InitHWSurfaces(this, current, surfaces_mem, surfaces_len);
 
+	/* Expose the back buffer as surface memory */
+	if ( current->flags & SDL_DOUBLEBUF ) {
+		this->screen = current;
+		DGA_FlipHWSurface(this, current);
+		this->screen = NULL;
+	}
+
 	/* Set the update rectangle function */
 	this->UpdateRects = DGA_DirectUpdate;