pkg/ast: fix out-of-bounds access

Scanner can access data out of bounds on bad input.
Also fix regression fuzz test to be able to detect the bug.

Author: dvyukov

pkg/ast/scanner.go

@@ -279,7 +279,6 @@ func (s *scanner) next() {
 		// Makes lots of things simpler as we always
 		// want to treat EOF as NEWLINE as well.
 		s.ch = '\n'
-		s.off++
 		return
 	}
 	if s.off > len(s.data) {

pkg/compiler/compiler_test.go

@@ -202,7 +202,7 @@ type D[e]l`,
 		"E",
 		"#",
 	} {
-		Fuzz([]byte(data))
+		Fuzz([]byte(data)[:len(data):len(data)])
 	}
 }