ResultRetriever: Clear m_dest_data between entries

ResultRetriever: Clear m_dest_data between entries

m_dest_data accidentally keeps its data between result entries so if an
entry has both stderr and dependency file data the stderr data will end
up in the dependency file as well. Fix this by clearing m_dest_data
properly.

Regression in 1cbaa89d1175c906bea48e40eb8f5e8c56b5afe4.

Fixes #629.