Fix for SPE-688 (crash when saving zip to low disk space)

src/slic3r/GUI/BackgroundSlicingProcess.cpp

@@ -113,10 +113,17 @@ public:
         zipstream(zipfile),
         pngstream(zipstream)
     {
-        if(!zipfile.IsOk())
+        if(!is_ok())
             throw std::runtime_error("Cannot create zip file.");
     }
 
+    ~LayerWriter() {
+        // In case of an error (disk space full) zipstream destructor would
+        // crash.
+        pngstream.clear();
+        zipstream.CloseEntry();
+    }
+
     inline void next_entry(const std::string& fname) {
         zipstream.PutNextEntry(fname);
     }
@@ -129,6 +136,10 @@ public:
         pngstream << arg; return *this;
     }
 
+    bool is_ok() const {
+        return pngstream.good() && zipstream.IsOk() && zipfile.IsOk();
+    }
+
     inline void close() {
         zipstream.Close();
         zipfile.Close();