Fix memory leak when failing to parse a quoted printable

Valgrind report:

    604 bytes in 1 blocks are definitely lost in loss record 8,010 of 10,179
      at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
      by 0x2BF73131: _i_NSData_QuotedPrintableCoding_dataByDecodingQuotedPrintableTransferEncoding (in /usr/lib/libNGExtensions.so.4.9.203)
      by 0x3375053E: _i_NSData_SOGoMailUtilities_bodyDataFromEncoding_ (in /usr/lib/GNUstep/SOGo/Mailer.SOGo/Mailer)
      by 0x34D23B46: _i_MAPIStoreMailMessage__setBodyContentFromRawData_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
      by 0x34D1DD0A: _i_MAPIStoreMailFolder__lookupMessage_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
      by 0x34CF4312: _i_MAPIStoreFolder__lookupMessageByURL_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
      by 0x34CF497A: _i_MAPIStoreFolder__openMessage_withMID_forWriting_inMemCtx_ (in /usr/lib/GNUstep/SOGo/SOGoBackend.MAPIStore/SOGoBackend)
      by 0x2A5D5013: sogo_folder_open_message (in /usr/lib/x86_64-linux-gnu/mapistore_backends/libMAPIStoreSOGo.so.1.0.0)