Fix new py-value.exp test case

The new test case in py-value.exp fails -- the code was changed to
throw ValueError, but the test still checks for TypeError.  This patch
fixes the problem.

I'm checking this in.  Tested on x86-64 Fedora 29.

gdb/testsuite/ChangeLog
2019-02-26  Tom Tromey  <tromey@adacore.com>

	* gdb.python/py-value.exp (test_value_from_buffer): Check for
	ValueError, not TypeError.