Fix: do not clear old XMP packet in case of an error encoding the new XMP data.

src/jpgimage.cpp

@@ -526,11 +526,15 @@ namespace Exiv2 {
                     }
                 }
                 if (writeXmpFromPacket() == false) {
-                    if (XmpParser::encode(xmpPacket_, xmpData_)) {
+                    std::string tmpXmpPacket;
+                    if (XmpParser::encode(tmpXmpPacket, xmpData_)) {
 #ifndef SUPPRESS_WARNINGS
-                        std::cerr << "Warning: Failed to encode XMP metadata.\n";
+                        std::cerr << "Error: Failed to encode XMP metadata.\n";
 #endif
                     }
+                    else {
+                        xmpPacket_ = tmpXmpPacket;
+                    }
                 }
                 if (xmpPacket_.size() > 0) {
                     // Write APP1 marker, size of APP1 field, XMP id and XMP packet