libgis: G_vasprintf bug fix

[HuidaeCho]

This PR fixes a G_vasprintf bug for #else of #ifdef HAVE_ASPRINTF.

From the man page of vsnprintf():

   Upon successful return, these functions return  the  number  of  bytes
   printed (excluding the null byte used to end output to strings).

   The  functions  snprintf() and vsnprintf() do not write more than size
   bytes (including the terminating null byte ('\0')).  If the output was
   truncated due to this limit, then the return value is  the  number  of
   characters (excluding the terminating null byte) which would have been
   written to the final string if enough space had been available.  Thus,
   a  return  value  of size or more means that the output was truncated.

1. We don't need to keep doubling size *= 2 because count is already the full length without \0 if it's greater than or equal to size.
2. ap cannot be reused in a for loop. It needs to be copied before its first use so we can use it again.