protobuf_c_message_unpack(): initialize tmp.length_prefix_len

this should silence Coverity #1153648, which complains because
tmp.length_prefix_len is uninitialized for certain wire types when
copied on line 2486:

    scanned_member_slabs[which_slab][in_slab_index++] = tmp;

protobuf-c/protobuf-c.c

@@ -2411,6 +2411,7 @@ protobuf_c_message_unpack(const ProtobufCMessageDescriptor *desc,
 		tmp.wire_type = wire_type;
 		tmp.field = field;
 		tmp.data = at;
+		tmp.length_prefix_len = 0;
 
 		switch (wire_type) {
 		case PROTOBUF_C_WIRE_TYPE_VARINT: {