Fix strdup implementation

[AvyChanna]

if condition in the given strdup was wrong.
memcpy should be done when malloc returns non-null value

For reference, here is the strdup inplementation in glibc -

/* Duplicate S, returning an identical malloc'd string.  */
char *
strdup (const char *s)
{
  size_t len = strlen (s) + 1;
  void *new = malloc (len);
  if (new == NULL)
    return NULL;
  return (char *) memcpy (new, s, len);
}