Fix out-of-bounds type stack read after rethrow in c-writer (#2798)

wasm2c on a valid exceptions module, ASan/debug build:

Assertion failed: (index < type_stack_.size()), function Write, file
c-writer.cc, line 1246.

Worked backwards from the StackVar read. The rethrow case in
`Write(const ExprList&)` ends in `break`, so the loop carries on and
emits the instructions after it. The validator has already marked those
unreachable (`OnRethrow` makes the stack polymorphic), so a valid module
can put operations there that pop operands which are not really on the
stack. StackVar then underflows `type_stack_` and reads off the end.
Release builds drop the assert and take the out-of-bounds read;
`DropTypes` does an out-of-bounds erase on the same tail.

br, br_table, return, unreachable, throw, throw_ref and the return_call
family all return here to stop walking the list. rethrow was the one
that fell through, so it now does the same.

Repro: `(func try nop catch_all rethrow 0 i32.store end)` assembled with
`--enable-exceptions`, then run through wasm2c.

Author: aizu-m

src/c-writer.cc

@@ -4187,7 +4187,9 @@ void CWriter::Write(const ExprList& exprs) {
         Write("wasm_rt_load_exception(", ex, "_tag, ", ex, "_size, ", ex, ");",
               Newline());
         WriteThrow();
-      } break;
+        // Stop processing this ExprList, since the following are unreachable.
+        return;
+      }
 
       case ExprType::Try: {
         const TryExpr& tryexpr = *cast<TryExpr>(&expr);

test/wasm2c/unreachable-after-rethrow.txt

@@ -0,0 +1,15 @@
+;;; TOOL: run-spec-wasm2c
+;;; ARGS*: --enable-exceptions
+(module
+  (memory 1)
+  (func (export "test")
+    try
+      nop
+    catch_all
+      rethrow 0
+      i32.store
+    end))
+(assert_return (invoke "test"))
+(;; STDOUT ;;;
+1/1 tests passed.
+;;; STDOUT ;;)