api: verify message size on receipt

api: verify message size on receipt

When a message is received, verify that it's sufficiently large to
accomodate any VLAs within message. To do that, we need a way to
calculate message size including any VLAs. This patch adds such
funcionality to vppapigen and necessary C code to use those to validate
message size on receipt. Drop messages which are malformed.

Type: improvement
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Change-Id: I2903aa21dee84be6822b064795ba314de46c18f4