Fix findbugs violations in liblldp

Fix findbugs violations in liblldp

- Method call passes null for non-null parameter
- Class defines clone() but doesn't implement Cloneable
- Clone method may return null
- clone method does not call super.clone()
- equals() method does not check for null argument
- Reliance on default encoding
- May expose internal representation by returning reference to mutable object
- Field is a mutable collection which should be package protected - malicious code vulnerability
- Field should be package protected - malicious code vulnerability
- Inefficient use of keySet iterator instead of entrySet iterator
- Boxing/unboxing to parse a primitive - use parseXXX method
- Redundant nullcheck of value known to be non-null

Change-Id: I6d551d31d618a1adfc877ac59f29d02727201df6
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>