Improvements based on static-analysis of source code

[DL6ER]

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This PR fixes several one-time memory leaks that might appear in failure cases of the database or memory allocation issues in several other places. None of them should be severe, however, we should obviously not leak memory in the first place. Furthermore, we check for the return value of a lot of routines that should not (but could) fail and enforce upper bounds for some DHCP options (relevant for dhcp-discover).