MasterTrack: Fix weak region running from the very start of the track

This is seen in some BBC HFEv3 images, eg. Crazy Tracer by Acornsoft
downloadable here:
https://drive.google.com/drive/mobile/folders/140xzJarpb3HX04MaqR_m8AzDoG3Bdyky

Also disallow ridiculously short weak regions to avoid any further crashes
in the weak-flux generator due to reasonable assumptions about minimum
lengths of the weak region and the overall track.

src/greaseweazle/track.py

@@ -168,9 +168,10 @@ def _flux(self, for_writeout, cue_at_index) -> Union[Flux, WriteoutFlux]:
         ticks_to_index = sum(bit_ticks)
 
         # Weak regions need special processing for correct flux representation.
-        for s,n in self.weak:
+        for s, n in self.weak:
+            if n < 2: continue # Too short to reliably weaken
             e = s + n
-            assert 0 < s < e <= bitlen
+            assert 0 <= s < e <= bitlen
             pattern = bitarray(endian="big")
             if n < 400 or self.force_random_weak:
                 # Short weak regions are written with no flux transitions.