Fix Witness debug display bug

Currently if the witness has zero elements or any of the individual
witnesses is empty we panic. Panic is caused by subtracting 1 from a
zero length.

Check the length is non-zero before subtracting 1, print `[]` if empty.

bitcoin/src/blockdata/witness.rs

@@ -68,21 +68,27 @@ fn fmt_debug(w: &Witness, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error>
     f.write_str("witnesses: [")?;
 
     let instructions = w.iter();
+    if instructions.len() > 0 {
         let last_instruction = instructions.len() - 1;
-
         for (i, instruction) in instructions.enumerate() {
             let bytes = instruction.iter();
+            if bytes.len() > 0 {
                 let last_byte = bytes.len() - 1;
-
                 f.write_str("[")?;
-
                 for (j, byte) in bytes.enumerate() {
                     write!(f, "{:#04x}", byte)?;
                     f.write_str(comma_or_close(j, last_byte))?;
                 }
-
+            } else {
+                // This is possible because the varint is not part of the instruction (see Iter).
+                write!(f, "[]")?;
+            }
             f.write_str(comma_or_close(i, last_instruction))?;
         }
+    } else {
+        // Witnesses can be empty because the 0x00 var int is not stored in content.
+        write!(f, "]")?;
+    }
 
     f.write_str(" }")
 }