lscpu: fix possibly undefined operation

lscpu: fix possibly undefined operation

With -Wall -Werror, compilation of lscpu.c fails with:

  Making all in sys-utils
  make[2]: Entering directory `/home/petr/upstream/util-linux/sys-utils'
  gcc -std=gnu99 -DHAVE_CONFIG_H -I. -I..  -include ../config.h -I../include -DLOCALEDIR=\"/usr/share/locale\"  -fsigned-char -Wall -Werror -MT lscpu.o -MD -MP -MF .deps/lscpu.Tpo -c -o lscpu.o lscpu.c
  lscpu.c: In function ‘print_parsable’:
  lscpu.c:971:7: error: operation on ‘p’ may be undefined [-Werror=sequence-point]
  cc1: all warnings being treated as errors

Fix by splitting the pointer increment to separate statement.

Signed-off-by: Petr Uzel <petr.uzel@suse.cz>