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.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-02-19  Tom Tromey  <tromey@adacore.com>
+
+	* python/python.c (do_start_initialization): Use XNEWVEC.  Remove
+	NULL check.
+
 2020-02-19  Maciej W. Rozycki  <macro@wdc.com>
 
 	* NEWS: Mention RISC-V GNU/Linux GDBserver support.

gdb/python/python.c

@@ -1679,12 +1679,7 @@ do_start_initialization ()
   std::string oldloc = setlocale (LC_ALL, NULL);
   setlocale (LC_ALL, "");
   progsize = strlen (progname.get ());
-  progname_copy = (wchar_t *) xmalloc ((progsize + 1) * sizeof (wchar_t));
-  if (!progname_copy)
-    {
-      fprintf (stderr, "out of memory\n");
-      return false;
-    }
+  progname_copy = XNEWVEC (wchar_t, progsize + 1);
   count = mbstowcs (progname_copy, progname.get (), progsize + 1);
   if (count == (size_t) -1)
     {