Return early in target_xfer_partial when LEN is zero.

Nowadays, argument LEN of to_xfer_partial can be zero in some cases,
and each implementation may do nothing and return zero, indicating
transfer is done.  That is fine.  However, when we change
to_xfer_partial to return target_xfer_status, we have to check every
return value of most of to_xfer_partial implementations, return
TARGET_XFER_DONE if return value is zero.

This patch simplifies this by checking LEN in target_xfer_partial, and
return 0 if LEN is zero.  Regression tested on x86_84-linux.  Is it
OK?

gdb:

2014-02-07  Yao Qi  <yao@codesourcery.com>

	* target.c (target_xfer_partial): Return zero if LEN is zero.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2014-02-07  Yao Qi  <yao@codesourcery.com>
+
+	* target.c (target_xfer_partial): Return zero if LEN is zero.
+
 2014-02-07  Yao Qi  <yao@codesourcery.com>
 
 	* auxv.c (procfs_xfer_auxv): Replace -1 with TARGET_XFER_E_IO.

gdb/target.c

@@ -1697,6 +1697,10 @@ target_xfer_partial (struct target_ops *ops,
 
   gdb_assert (ops->to_xfer_partial != NULL);
 
+  /* Transfer is done when LEN is zero.  */
+  if (len == 0)
+    return 0;
+
   if (writebuf && !may_write_memory)
     error (_("Writing to memory is not allowed (addr %s, len %s)"),
 	   core_addr_to_string_nz (offset), plongest (len));