Fix resolution of delegate labels (#1675)

This the label resolution code. Example from the spec (https://github.com/WebAssembly/exception-handling/blob/master/proposals/exception-handling/Exceptions.md):

```
try $l1
  try
    call $foo
  delegate $l1  ;; (= delegate 0)
catch
  ...
catch_all
  ...
end
```

The label `$l1` should resolve to `0` in the delegate target, which is 1 less than how typical block constructs work. The current code mis-resolves this to `1`. It should also error in cases like the following:

```
try $l1
  nop
delegate $l1
```

Because it's not possible to represent the label `$l1` in the binary format, as a `try-delegate` cannot target itself.

1 files changed, 15 insertions, 0 deletions

diff --git a/test/parse/bad-delegate-label.txt b/test/parse/bad-delegate-label.txt
new file mode 100644
index 00000000..bff8f05c
--- /dev/null
+++ b/test/parse/bad-delegate-label.txt
@@ -0,0 +1,15 @@
+;;; TOOL: wat2wasm
+;;; ARGS: --enable-exceptions
+;;; ERROR: 1
+(module
+  (func
+    try $t
+      nop
+    delegate $t
+  )
+)
+(;; STDERR ;;;
+out/test/parse/bad-delegate-label.txt:8:14: error: undefined label variable "$t"
+    delegate $t
+             ^^
+;;; STDERR ;;)