Use checked_sub

Recently we "if" guarded subtraction manually using `> 0`, we can better
convey the meaning by using `checked_sub` and pattern match on the
option.

Refactor only, no logic changes.

bitcoin/src/blockdata/witness.rs

@@ -68,27 +68,31 @@ 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;
+    match instructions.len().checked_sub(1) {
+        Some(last_instruction) => {
             for (i, instruction) in instructions.enumerate() {
                 let bytes = instruction.iter();
-            if bytes.len() > 0 {
-                let last_byte = bytes.len() - 1;
+                match bytes.len().checked_sub(1) {
+                    Some(last_byte) => {
                         f.write_str("[")?;
                         for (j, byte) in bytes.enumerate() {
                             write!(f, "{:#04x}", byte)?;
                             f.write_str(comma_or_close(j, last_byte))?;
                         }
-            } else {
+                    }
+                    None => {
                         // 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 {
+        }
+        None => {
             // Witnesses can be empty because the 0x00 var int is not stored in content.
             write!(f, "]")?;
         }
+    }
 
     f.write_str(" }")
 }