fix error checking of asprintf

fix error checking of asprintf

see man asprintf
  [...]
  RETURN VALUE
   When  successful, these  functions return the number of bytes printed, just like sprintf(3).
   If memory allocation wasn't possible, or some other error occurs, these functions will return -1,
   and the contents of strp is undefined.

Signed-off-by: Frank Meerkötter <frank@meerkoetter.org>