[sanitizer] scanf interceptor: fix write size for %mc/%mC/%mS

When the optional assignment-allocation character 'm' (Extension to the
ISO C standard) is present, we currently use internal_strlen(buf)+1 for
all of cCsS[ (D85350). Fix cCS to use the correct size.

Fix https://github.com/llvm/llvm-project/issues/61768

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D158485