Fix '-data-read-memory-bytes' typo/assertion

This patch fixes a typo in target.c:read_memory_robust, where
it calls read_whatever_is_readable with the function arguments
in the wrong order.  Depending on the address being read, it
can cause an xmalloc with a huge size, resulting in an assertion
failure, or just read something other than what was requested.

The problem only arises when GDB is handling an MI
"-data-read-memory-bytes" request and the initial target_read returns
an error status.  Note that read_memory_robust is only called from
the MI code.

gdb/ChangeLog:

	* gdb/target.c (read_memory_robust): Call
	read_whatever_is_readable with arguments in the correct order.