AVRO-3221: Verify NUL termination on set_string_len and make buffer more compact

[ashdnazg]

Previously all size bytes were copied from the given string,
including the required NUL terminator, and then another NUL terminator
was appended in avro_raw_string_set_length.
Now the existence of the NUL terminator is verified, allowing to
catch programmer mistakes and then only size - 1 bytes are copied
before appending the NUL terminator.

Make sure you have checked all steps below.

Jira

• My PR addresses the following Avro Jira issues and references them in the PR title. For example, "AVRO-1234: My Avro PR"
  • https://issues.apache.org/jira/browse/AVRO-3221
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Test that setting a non-null terminated string fails.

Commits

• My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"