Log Message:
Update yara to version 3.6.3

From the release notes for version 3.6.3:
* BUGFIX: Heap overflow (4a342f0)
* BUGFIX: Off-by-one NULL write in stack buffer (964d6c0)
* BUGFIX: Multiple issues in "dotnet" module (f40c14c, fc35e5f)

From the release notes for version 3.6.2:

* Increase RE_MAX_AST_LEVELS from 2000 to 6000.
* BUGFIX: Buffer overrun in regexp engine (issue #678)
* BUGFIX: Null pointer dereference in regexp engine (issue #682).

XXX pullup (security fixes)