libbinder: readCString: no ubsan sub-overflow

Bug: 131859347
Test: fuzzer
Change-Id: I95a0f59684a172925f1eab97ff21e5d14bc79cc8
Merged-In: I95a0f59684a172925f1eab97ff21e5d14bc79cc8
(cherry picked from commit d0d4b584fc294d2c124385644099852918416344)
1 file changed