[sanitizer] Fix handling of %n in scanf interceptor.

%n does not increase the input item count. The new code emits writes to %n
arguments even if it has run out of input items.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@188069 91177308-0d34-0410-b5e6-96231b3b80d8