BUG/MEDIUM: sample: reject the deprecated protobuf group wire types

BUG/MEDIUM: sample: reject the deprecated protobuf group wire types

protobuf_field_lookup() dispatches on the field's wire type through
protobuf_parser_defs[], but the entries for wire types 3 and 4
(START_GROUP/STOP_GROUP, deprecated) are zero-initialized:

    [PBUF_TYPE_START_GROUP     ] = {
            /* XXX Deprecated XXX */
    },
    [PBUF_TYPE_STOP_GROUP      ] = {
            /* XXX Deprecated XXX */
    },

while the only validation is the upper bound on the table:

    if (wire_type >= sizeof(protobuf_parser_defs) / sizeof(protobuf_parser_defs[0]))
            return 0;

A field using wire type 3 or 4 therefore passes the check and reaches a
call through the NULL .skip/.smp_store pointer: a request body
consisting of the single byte 0x0b (field 1, wire type 3) crashes the
process at pc=0. Any configuration routing a client body through the
"protobuf" converter (e.g. "http-request set-var(txn.f)
req.body,protobuf(1)") makes this remotely and unauthenticatedly
triggerable.

Let's reject wire types whose parser definition is missing and return 0
("field not found"), in line with the function's existing failure
contract.

The protobuf converter first appeared in 2.2; this should be backported
to all supported versions.