37: `curl_mprintf` `%F` format specifier not consumed, causing variadic argument desynchronization

`docs/libcurl/curl_mprintf.md` documents `%F` as a conversion that consumes a `double`, alongside `%f`. However, `parse_conversion()` in `lib/mprintf.c` has no `F` case — it falls through to the default branch and returns without assigning an input slot. In a sequential format like `"%F%s"`, the …

case 'f':
  type = MTYPE_DOUBLE;
  break;
// ... e, E, g, G handled ...
default:
  /* invalid instruction, disregard and continue */
  return TRUE;