imult64 - integer multiplication with overflow checking

       imult64 does c=a*b and returns 1.

       In case of an integer overflow, imult64 leaves c alone and returns 0.

       imult64 - integer multiplication with overflow checking

imult16(3), imult32(3), umult64(3)

                                                                                                      imult64(3)

#include<libowfat/safemult.h>

       int imult64(int64 a,int64 b,int64* c);