descriptor: require an extra byte to generate raw expressions

raw() with no arguments generates a zero length script. Ensure we tell
the caller they need at least one byte of generation space for this
case, otherwise they would be passing an invalid (empty) output buffer.

Author: jgriffiths

src/descriptor.c

@@ -2546,8 +2546,12 @@ static int node_generation_size(const ms_node *node, size_t *total)
             /* max of p2pk, p2pkh, p2wpkh, or p2sh-p2wpkh */
             *total += WALLY_SCRIPTPUBKEY_P2PKH_LEN;
             break;
-        case KIND_DESCRIPTOR_ADDR:
         case KIND_DESCRIPTOR_RAW:
+            /* Add an extra byte to handle 'raw()' which results in nothing,
+             * as empty output buffers cannot be passed to descriptor calls.
+             */
+            *total += 1;
+        case KIND_DESCRIPTOR_ADDR:
             /* No-op */
             break;
         case KIND_DESCRIPTOR_RAW_TR: