fix php warning in dol_convertToWord when negative numbers

[FHenry]

In credti note numbers/figures is negative

so
$num = (int) $TNum[0]
give $num = -360
and latter

$num = substr('00'.$num, -$max_length);
$num_levels = str_split($num, 3);

$num_levels give ['00-','360']

'00-' is not a numric values