Fix error C4700 compiling with Visual C++

With Visual C++ 2012, error C4700 is issued when compiled with either /Od(default in Debug mode) or /Ox:
fossa.c(575): error 4700: local variable 'sa' used without having been initialized.
fossa.c(582): error 4700: local variable 'sa' used without having been initialized.

make an explicit zero memory operation as my solution.