Improve lexing of invalid annotations (#2150)

This had been crashing even with annotations disabled.
Adds a regression test.

This was missed in #2001 when updating the lexer to match
the updated spec (WebAssembly/spec#1499).

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=53935

Author: keithw

src/wast-lexer.cc

@@ -70,7 +70,7 @@ Token WastLexer::GetToken() {
           }
           return BareToken(TokenType::Eof);
         } else if (MatchString("(@")) {
-          ReadReservedChars();
+          GetIdToken();
           // offset=2 to skip the "(@" prefix
           return TextToken(TokenType::LparAnn, 2);
         } else {

test/regress/bad-annotation.txt

@@ -0,0 +1,12 @@
+;;; TOOL: wat2wasm
+;;; ERROR: 1
+(@"annotation
+
+(;; STDERR ;;;
+out/test/regress/bad-annotation.txt:3:1: error: annotations not enabled: "annotation
+(@"annotation
+^^^^^^^^^^^^^
+out/test/regress/bad-annotation.txt:3:1: error: unexpected token "Invalid", expected a module field or a module.
+(@"annotation
+^^^^^^^^^^^^^
+;;; STDERR ;;)