PR tdep/17379: Fix internal-error when stack pointer is invalid.

PR tdep/17379: Fix internal-error when stack pointer is invalid.

The problem is that rs6000_frame_cache attempts to read the stack backchain via
read_memory_unsigned_integer, which throws an exception if the stack pointer is
invalid.  With this patch, it calls safe_read_memory_integer instead, which
doesn't throw an exception and allows for safe handling of that situation.

gdb/ChangeLog
2014-09-12  Edjunior Barbosa Machado  <emachado@linux.vnet.ibm.com>
	    Ulrich Weigand  <uweigand@de.ibm.com>

	PR tdep/17379
	* rs6000-tdep.c (rs6000_frame_cache): Use safe_read_memory_integer
	instead of read_memory_unsigned_integer.

gdb/testcase/ChangeLog
2014-09-12  Edjunior Barbosa Machado  <emachado@linux.vnet.ibm.com>

	PR tdep/17379
	* gdb.arch/powerpc-stackless.S: New file.
	* gdb.arch/powerpc-stackless.exp: New file.