net: dns: Check query index in dns_read

This commit adds a check, in the `dns_read` function, before
dereferencing the query index returned by the `dns_validate_msg`
function.

This fixes the warnings generated by the GCC 12 such as:

  error: array subscript -1 is below array bounds of
  'struct dns_pending_query[5]' [-Werror=array-bounds]

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>