modpost: do not make find_tosym() return NULL

As mentioned in commit 39758650 ("modpost: Add '.ltext' and
'.ltext.*' to TEXT_SECTIONS"), modpost can result in a segmentation
fault due to a NULL pointer dereference in default_mismatch_handler().

find_tosym() can return the original symbol pointer instead of NULL
if a better one is not found.

This fixes the reported segmentation fault.

Fixes: a23e7584 ("modpost: unify 'sym' and 'to' in default_mismatch_handler()")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>