Check to make sure we have an input file.

mcopy.cpp

@@ -152,6 +152,11 @@ int main(int argc, char **argv) {
         exit(1);
     }
 
+    if (argc < 4) {
+        std::cout << "mcopy: You must specify a file to copy.\n";
+        exit(1);
+    }
+
     for (int i{1}; i < ac; i++) {
         std::ifstream f(av[i]);