python/xc: Fix multiple issues in pyxc_readconsolering()

python/xc: Fix multiple issues in pyxc_readconsolering()

Don't leak a 16k allocation if PyArg_ParseTupleAndKeywords() or the first
xc_readconsolering() fail.  It is trivial to run throught the processes memory
by repeatedly passing junk parameters to this function.

In the case that the call to xc_readconsolering() in the while loop fails,
reinstate str before breaking out, and passing a spurious pointer to free().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Coverity-IDs: 1054984 1055906
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Xen Coverity Team <coverity@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>