liblog: null terminate log_msg

Also, use LOGGER_ENTRY_MAX_LEN instead of sizeof(log_msg) for ensuring
that the read did not overflow, since sizeof(log_msg) is one byte
longer to allow for placing a null terminator already.

Bug: 148966104
Test: liblog unit tests
Change-Id: Id4fe8aa5da44e9d62f59b6541f157a4e7b038d07