wifi: wext-core: Fix -Wstringop-overflow warning in ioctl_standard_iw_point()

-Wstringop-overflow is legitimately warning us about extra_size
pontentially being zero at some point, hence potenially ending
up _allocating_ zero bytes of memory for extra pointer and then
trying to access such object in a call to copy_from_user().

Fix this by adding a sanity check to ensure we never end up
trying to allocate zero bytes of data for extra pointer, before
continue executing the rest of the code in the function.

Address the following -Wstringop-overflow warning seen when built
m68k architecture with allyesconfig configuration:
                 from net/wireless/wext-core.c:11:
In function '_copy_from_user',
    inlined from 'copy_from_user' at include/linux/uaccess.h:183:7,
    inlined from 'ioctl_standard_iw_point' at net/wireless/wext-core.c:825:7:
arch/m68k/include/asm/string.h:48:25: warning: '__builtin_memset' writing 1 or more bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]
   48 | #define memset(d, c, n) __builtin_memset(d, c, n)
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/uaccess.h:153:17: note: in expansion of macro 'memset'
  153 |                 memset(to + (n - res), 0, res);
      |                 ^~~~~~
In function 'kmalloc',
    inlined from 'kzalloc' at include/linux/slab.h:694:9,
    inlined from 'ioctl_standard_iw_point' at net/wireless/wext-core.c:819:10:
include/linux/slab.h:577:16: note: at offset 1 into destination object of size 0 allocated by '__kmalloc'
  577 |         return __kmalloc(size, flags);
      |                ^~~~~~~~~~~~~~~~~~~~~~

This help with the ongoing efforts to globally enable
-Wstringop-overflow.

Link: https://github.com/KSPP/linux/issues/315


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/ZItSlzvIpjdjNfd8@work


Signed-off-by: Johannes Berg <johannes.berg@intel.com>