FDFAnnotationStamp fails to parse some appearance streams

Some stamps still cause issues in 2.0.15 when their appearance streams are parsed.

I attached two XFDF files which produces this error when I merge it :

Apr 12, 2019 7:46:34 AM org.apache.pdfbox.util.Hex decodeHex
SCHWERWIEGEND: Can't parse
0, aborting decode
java.lang.NumberFormatException: For input string: "
0"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Integer.parseInt(Integer.java:569)
at org.apache.pdfbox.util.Hex.decodeHex(Hex.java:279)
at org.apache.pdfbox.pdmodel.fdf.FDFAnnotationStamp.parseStreamElement(FDFAnnotationStamp.java:253)
at org.apache.pdfbox.pdmodel.fdf.FDFAnnotationStamp.parseStampAnnotationAppearanceXML(FDFAnnotationStamp.java:175)
at org.apache.pdfbox.pdmodel.fdf.FDFAnnotationStamp.<init>(FDFAnnotationStamp.java:134)
at org.apache.pdfbox.pdmodel.fdf.FDFDictionary.<init>(FDFDictionary.java:211)
at org.apache.pdfbox.pdmodel.fdf.FDFCatalog.<init>(FDFCatalog.java:63)
at org.apache.pdfbox.pdmodel.fdf.FDFDocument.<init>(FDFDocument.java:90)

it looks like the attribute value contains a LF/CR. When this error occurs, the appearance is missing and only the outline of the stamp is shown when the user selects the element.

I found two of the default stamps in Adobe Reader (german version) to cause this

- SBNotForPublicRelease ("Nicht zur Veröffentlichung", bad-stamp-2.0.15.xfdf)

- SBPreliminaryResults ("Vorläufige Ergebnisse", bad-stamp2-2.0.15.xfdf)