fix: comp-3 codec should be tolerant to invalid buffers

[adrienaury]

Example

• PIC Format : S9(06)
• Buffer : F0 40 40 40

Decode

• Expected result : Something
• Actual Result : #F04040.

Encode

• Expected result : F0 40 40 40
• Actual Result : Error !

Possible mapping for sign nibble

Nibble	Character
0	SP
1	!
2	"
3	#
4	$
5	%
6	&
7	'
8	(
9	)
A	*
B	,
C	+
D	-
E	/
F