Microblaze: Reject invalid target descriptions

We currently validate the target description, but then forget to
reject it if found invalid.

Tested that incorrect descriptions are rejected and GDB warns about
them.

Tested the Microblaze Design with and without stack-protect registers.
The gdb command "info registers" displayed the register correctly.  If
a stack protect design is not selected, only core registers are
displayed.  When the stack-protect registers are selected in the
design, the core registers along with stack-protect registers are
displayed.

gdb/
2014-10-10  Ajit Agarwal  <ajitkum@xilinx.com>

	* microblaze-tdep.c (microblaze_gdbarch_init): If the description
	isn't valid, release the tdesc arch data and return NULL.