mesa: handle GL_NONE draw buffers correctly in glClear

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/clear.c

@@ -179,7 +179,11 @@ _mesa_Clear( GLbitfield mask )
       if (mask & GL_COLOR_BUFFER_BIT) {
          GLuint i;
          for (i = 0; i < ctx->DrawBuffer->_NumColorDrawBuffers; i++) {
-            bufferMask |= (1 << ctx->DrawBuffer->_ColorDrawBufferIndexes[i]);
+            GLint buf = ctx->DrawBuffer->_ColorDrawBufferIndexes[i];
+
+            if (buf >= 0) {
+               bufferMask |= 1 << buf;
+            }
          }
       }