Adjust modbus register array parsing.

For modbus message types that include variable amount of register values
(uint16[]), setting a &length attribute without an explicit array size
could trigger a parsing assertion since it allows for the "element" data
pointer to travel past the "end of data" (e.g. when &length is odd).
This is changed to now give both an array size and &length to earlier
terminate the parsing of elements before the assert is checked and
so a single out-of-bound check can be done for the entire array
(leaving off &length causes an out-of-bound check for each element).

Added another parameter to modbus events that carry register arrays to
the script-layer which indicates the associated byte count from the
message (allowing for invalid values to be detected):

    modbus_read_holding_registers_response
    modbus_read_input_registers_response
    modbus_write_multiple_registers_request
    modbus_read_write_multiple_registers_request
    modbus_read_write_multiple_registers_response
    modbus_read_fifo_queue_response

testing/btest/Baseline/scripts.base.protocols.modbus.register_parsing/modbus.log

@@ -0,0 +1,12 @@
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	modbus
+#open	2012-11-12-21-51-15
+#fields	ts	uid	id.orig_h	id.orig_p	id.resp_h	id.resp_p	func	exception
+#types	time	string	addr	port	addr	port	string	string
+1342774775.305761	UWkUyAuUGXf	10.1.1.234	51411	10.10.5.104	502	READ_INPUT_REGISTERS	-
+1342775209.493066	arKYeMETxOg	10.1.1.234	51411	10.10.5.104	502	READ_INPUT_REGISTERS	-
+1342776371.617757	nQcgTWjvg4c	10.1.1.234	51411	10.10.5.104	502	READ_INPUT_REGISTERS	-
+#close	2012-11-12-21-51-15