…c check

56a461f wallet: fix buffer over-read in SQLite file magic check (Sebastian Falbesoner)

Pull request description:

  Looking at our new SQLite database code, I noticed that there is a potential problem in the method `IsSQLiteFile()`:  If there is no terminating zero within the first 16 bytes of the file, the `magic` buffer would be over-read in the `std::string` constructor for `magic_str`. Fixed by using the "from buffer" variant of the string ctor (that also takes a size) rather than the "from c-string" variant (see http://www.cplusplus.com/reference/string/string/string/).

  The behaviour can be reproduced by the following steps:
  * Creating a file of at least 512 bytes in size (to pass the minimum size check) that doesn't contain zero bytes in the magic area, e.g. simply:
  `$ python3 -c "print('A'*512)" > /tmp/corrupt_wallet`
  * Showing content and size of the `magic_str` string in case the magic check fails
  * Create a simple unit test that simply calls `IsSQLiteFile` with the corrupt wallet file
  * Run the unit test and see the random gibberish output of `magic_str` after 16 `A`s :-)

  Or, TLDR variant, just get the branch https://github.com/theStack/bitcoin/tree/reproduce_sqlite_magic_overread, compile unit Tests and run the script `./reproduce_sqlite_magic_overread.sh`.

  Note that this is the minimal diff, probably it would be better to avoid `std::string` at all in this case and just use `memcmp`, strings that include null bytes are pretty confusing.

ACKs for top commit:
  promag:
    Code review ACK 56a461f.
  practicalswift:
    ACK 56a461f: patch looks correct
  achow101:
    ACK 56a461f

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