bitcoin-utils: fix encoding of CompactSize integers

Encoding an atom as a Compact fails when a = 0: a 4-byte CompactSize is
emitted.

pkg/base-dev/lib/bitcoin-utils.hoon

@@ -143,6 +143,7 @@
     |=  a=@
     ^-  hexb
     =/  l=@  (met 3 a)
+    ?:  =(l 0)  1^a
     ?:  =(l 1)  1^a
     ?:  =(l 2)  (cat:byt ~[1^0xfd (flip:byt 2^a)])
     ?:  (lte l 4)  (cat:byt ~[1^0xfe (flip:byt 4^a)])