x86/ioremap: prevent additions against the NULL pointer

x86/ioremap: prevent additions against the NULL pointer

This was reported by clang UBSAN as:

UBSAN: Undefined behaviour in arch/x86/mm.c:6297:40
applying zero offset to null pointer
[...]
Xen call trace:
    [<ffff82d040303662>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xc0
    [<ffff82d040304aa3>] F __ubsan_handle_pointer_overflow+0xcb/0x100
    [<ffff82d0406ebbc0>] F ioremap_wc+0xc8/0xe0
    [<ffff82d0406c3728>] F video_init+0xd0/0x180
    [<ffff82d0406ab6f5>] F console_init_preirq+0x3d/0x220
    [<ffff82d0406f1876>] F __start_xen+0x68e/0x5530
    [<ffff82d04020482e>] F __high_start+0x8e/0x90

Fix bt_ioremap() and ioremap{,_wc}() to not add the offset if the returned
pointer from __vmap() is NULL.

Fixes: d0d4635d034f ('implement vmap()')
Fixes: f390941a92f1 ('x86/DMI: fix table mapping when one lives above 1Mb')
Fixes: 81d195c6c0e2 ('x86: introduce ioremap_wc()')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>