mesa: Include null terminator in GL_DEBUG_NEXT_LOGGED_MESSAGE_LENGTH.

mesa: Include null terminator in GL_DEBUG_NEXT_LOGGED_MESSAGE_LENGTH.

From the KHR_debug spec:
"Applications can query the number of messages currently in the log by
 obtaining the value of DEBUG_LOGGED_MESSAGES, and the string length
 (including its null terminator) of the oldest message in the log
 through the value of DEBUG_NEXT_LOGGED_MESSAGE_LENGTH."

Because we weren't including the null terminator, many dEQP tests
called glGetDebugMessageLog with a bufSize parameter that was 1 too
small, and unable to contain the message, so we skipped returning it,
failing many cases.

Fixes 298 dEQP-GLES31.functional.debug.* tests.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Stephane Marchesin <stephane.marchesin@gmail.com>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>