net: dhcpv4: client: Prevent asserting on malformed message

In case the received DHCP message is malformed and contains invalid
message type, the code responsible for matching message type with a
string would assert. This shouldn't be the case that external conditions
(like receiving malformed packet) trigger asserts in the system.
Therefore modify that code, to return "invalid" string in such case
instead.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>