Possible fix to lavc crash.

Originally committed to SVN as r1984.

aegisub/lavc_keyframes.cpp

@@ -43,7 +43,7 @@
 
 ///////////////
 // Constructor
-LAVCKeyFrames::LAVCKeyFrames(const Aegisub::String& filename) 
+LAVCKeyFrames::LAVCKeyFrames(const Aegisub::String filename) 
  : file(0), codecContext(0), codec(0), stream(0), frame(0), 
    streamN(-1) {
 	// Open LAVCFile

aegisub/lavc_keyframes.h

@@ -48,7 +48,7 @@ class LAVCKeyFrames {
 
 		int streamN;					// Stream index
 	public:
-		LAVCKeyFrames(const Aegisub::String& filename);
+		LAVCKeyFrames(const Aegisub::String filename);
 		~LAVCKeyFrames();
 		wxArrayInt GetKeyFrames();
 };