dm table: don't copy from a NULL pointer in realloc_argv()

For the first call to realloc_argv() in dm_split_args(), old_argv is
NULL and size is zero. Then memcpy is called, with the NULL old_argv
as the source argument and a zero size argument. AFAIK, this is
undefined behavior and generates the following warning when compiled
with UBSAN on ppc64le:

In file included from ./arch/powerpc/include/asm/paca.h:19,
                 from ./arch/powerpc/include/asm/current.h:16,
                 from ./include/linux/sched.h:12,
                 from ./include/linux/kthread.h:6,
                 from drivers/md/dm-core.h:12,
                 from drivers/md/dm-table.c:8:
In function 'memcpy',
    inlined from 'realloc_argv' at drivers/md/dm-table.c:565:3,
    inlined from 'dm_split_args' at drivers/md/dm-table.c:588:9:
./include/linux/string.h:345:9: error: argument 2 null where non-null expected [-Werror=nonnull]
  return __builtin_memcpy(p, q, size);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/md/dm-table.c: In function 'dm_split_args':
./include/linux/string.h:345:9: note: in a call to built-in function '__builtin_memcpy'

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>