[sanitizer] Handle null in internal_sigaction_syscall

Summary:
Adds support for nullptr as the action parameter to
internal_sigaction_syscall().

Reviewers: aizatsky

Subscribers: kubabrecka, vitalybuka, zhaoqin, kcc, eugenis, llvm-commits

Differential Revision: http://reviews.llvm.org/D22061

llvm-svn: 274665