Fix Python unwinder frames regression

Fix Python unwinder frames regression

The gdb.python/py-unwind.exp test is crashing GDB / leaving core dumps
in the test dir, even though it all passes cleanly.  The crash is not
visible in gdb.sum/gdb.log because it happens as side effect of the
"quit" command, while flushing the frame cache.

The problem is simply a typo in a 'for' loop's condition, introduced
by a recent change [4fa847d78edd ("Remove MAX_REGISTER_SIZE from
py-unwind.c")], resulting in infinite loop / double-free.

The new test exposes the crash, like:

 Running src/gdb/testsuite/gdb.python/py-unwind.exp ...
 ERROR: Process no longer exists

gdb/ChangeLog:
2017-07-06  Pedro Alves  <palves@redhat.com>

	* python/py-unwind.c (pyuw_dealloc_cache): Fix for loop condition.

gdb/testsuite/ChangeLog:
2017-07-06  Pedro Alves  <palves@redhat.com>

	* gdb.python/py-unwind.exp: Test flushregs.