Fix leak in KMime::Content::decodedContent()
ASAN trace:
Indirect leak of 12 byte(s) in 3 object(s) allocated from:
#0 0x52a000 in operator new(unsigned long) (/home/kfunk/devel/install/kf5/bin/kmail+0x52a000) #1 0x7f294f8f13b3 in KCodecs::Decoder::Decoder(KCodecs::Codec::NewlineType) /home/kfunk/devel/src/kf5/kcodecs/src/kcodecs.cpp:770:9 #2 0x7f294f8f4ba4 in KCodecs::Base64Decoder::Base64Decoder(KCodecs::Codec::NewlineType) /home/kfunk/devel/src/kf5/kcodecs/src/kcodecsbase64.cpp:88:11 #3 0x7f294f8f3e6c in KCodecs::Base64Codec::makeDecoder(KCodecs::Codec::NewlineType) const /home/kfunk/devel/src/kf5/kcodecs/src/kcodecsbase64.cpp:170:16 #4 0x7f294baba7e4 in KMime::Content::decodedContent() /home/kfunk/devel/src/kf5/kmime/src/kmime_content.cpp:345:48