[BUGFIX] Unnecessary warning in css_styled_content (division by zero)

If tt_content.image.20.maxW is not set, PHP throws a warning about
division by zero. This is not necessary. After that the variable
$scale is not set. This patch checks if $netW is greater zero and
otherwise the else condition will set $scale to 1.

Change-Id: Icd876b33d543080e486e0184c8af34c9a3831738
Fixes: #36777
Releases: 6.0, 4.7, 4.6
Reviewed-on: http://review.typo3.org/10928
Reviewed-by: Michael Klapper
Tested-by: Michael Klapper
Reviewed-by: Philipp Gampe
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer