ips_layer: Fix inaccuracies with comments and flags

Specifically bugs/crashes that arise when putting them in positions that are legal but not typical, such as midline, between patch data, or between patch records.

src/core/file_sys/ips_layer.h

@@ -22,6 +22,7 @@ public:
     VirtualFile Apply(const VirtualFile& in) const;
 
 private:
+    void ParseFlag(const std::string& flag);
     void Parse();
 
     bool valid = false;