Error gracefully when requested to write a too-large scp image

This prevents a crash when more tracks were recorded than fit in an
scp image. When I first encountered a problem doing an 84-track operation,
this led to a segfault later on since memory was written past the end
of _fileheader, and (I think) stomped on a pointer.

lib/fluxsink/scpfluxsink.cc

@@ -85,6 +85,12 @@ public:
 		trackdataWriter.seekToEnd();
 		int strack = strackno(cylinder, head);
 
+                if (strack >= std::size(_fileheader.track)) {
+                    std::cout << fmt::format("SCP: cannot write track {} head {}, "
+                            "there are not not enough Track Data Headers.\n",
+                            cylinder, head);
+                    return;
+                }
 		ScpTrack trackheader = {0};
 		trackheader.header.track_id[0] = 'T';
 		trackheader.header.track_id[1] = 'R';