[sanitizer] use uptr to store the result of internal_strlen/internal_strnlen in printf_common

The return type of `internal_strlen()` is 'uptr', but in `printf_common()` we store the result of `internal_strlen()` into an 'int' type variable.
When the result value of `internal_strlen()` is larger than the largest possible value of 'int' type, the implicit conversion from 'uptr' to 'int' will change the result value to a negative value.

Without this change, asan reports a false positive negative-size-param in the added testcase.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D157266