Fixed bug where sendfile will fail with EINVAL

Not all Unix's come with the same implementation of sendfile.
This change attempts it, but if it fails it does normal sys read and
write. Sendfile is way faster than sys read and write, so it is worth
attempting

src/cat.c

@@ -22,7 +22,8 @@ void throw_error(const char* extra_msg, const int exit_code){
 }
 void cat(const int fd, const char no_buf, size_t size){
 /* reads from the file descriptor into buf, until zero bytes are read  */
-	if(fd == STDIN_FILENO || size > MAX_SENDFILE_LINUX){
+	//tries sendfile if available (since it is faster, if not, do sys read & write)
+	if(fd == STDIN_FILENO || size > MAX_SENDFILE_LINUX || sendfile(STDOUT_FILENO, fd, 0, size) != size){
 		size_t r = 0;
 		if(no_buf) size = 1;
 		void *buf = (void*)malloc(size);
@@ -31,8 +32,8 @@ void cat(const int fd, const char no_buf, size_t size){
 			if(write(STDOUT_FILENO, buf, r) != r) throw_error(NULL, 3); //if writing stops for some reason
 		if(r == -1) throw_error(NULL, 1); //if read(2) throws an error
 		free(buf);
-	} else //if we are given a file, use the sendfile syscall for better speed
-		if(sendfile(STDOUT_FILENO, fd, 0, size) != size) throw_error("In sendfile", 1);
+	} //else 
+	//	throw_error("Unable to cat file.", 1);
 }
 void get_file(const char* file_name, const char no_buf){
 /* tries to open the user's file, then calls cat() with its file pointer */