sql/parser: fix memory leak of scanned comments

This commit fixes a memory leak that cause a session's
`scanner.Scanner`, which is embedded in a `pgwire.conn` via a
`parser.Parser`, to retain previously scanned SQL comments in all parsed
SQL statements. The scanner's slice of comments would continue to grow
as it parsed new SQL strings with comments, as long as the session
remained open. Also, the slice of comments holds references to the bytes
in `pgwire` read buffers, preventing those buffers from being GC'd.

Fixes cockroachdb#127710

Release note: None

pkg/sql/scanner/scan.go

@@ -98,6 +98,7 @@ func (s *Scanner) Init(str string) {
 // where we reuse a Scanner).
 func (s *Scanner) Cleanup() {
 	s.bytesPrealloc = nil
+	s.Comments = nil
 }
 
 func (s *Scanner) allocBytes(length int) []byte {