Don't update target_dcache if it is not initialized

Don't update target_dcache if it is not initialized

After previous patch, 'target_dcache' is initialized lazily.  It is
possible that 'target_dcache' is still NULL when GDB writes to memory.
In this case, update to 'target_dcache' can be skipped.

gdb:

2013-11-20  Yao Qi  <yao@codesourcery.com>

	* target.c (memory_xfer_partial_1): Update 'target_dcache' if
	it is initialized.