Remove useless NULL check in python.c

Remove useless NULL check in python.c

I noticed that do_start_initialization, in python.c, checks the result
of xmalloc.  However, xmalloc cannot fail, so this check is useless.
This patch also changes the code to use XNEWVEC.

gdb/ChangeLog
2020-02-19  Tom Tromey  <tromey@adacore.com>

	* python/python.c (do_start_initialization): Use XNEWVEC.  Remove
	NULL check.