You probably want to pad it with 0 bytes. Fill your newtitle buffer with zeroes before copying the string into it, then write that buffer to the file:
char newtitle[80];
memset(newtitle, '\0', 80);
strncpy(newtitle, title, 80);
fwrite(newtitle, sizeof(char), 80, fp);
You can pass whatever ASCII character you want into memset(), to pad it with. But with binary files you'll generally use '\0'
.