Initialize sigset in asan_interceptors (#69502)

When compiling for Darwin, sigset is not initialized. 

When -Werror,-Wuninitialized-const-reference are enabled we see the
error:
asan_interceptors.cpp:260:38: error: variable 'sigset' is uninitialized
when passed as a const reference argument here
[-Werror,-Wuninitialized-const-reference]

This fixes the error