promql/parser: Cleanup generatedParserResult accross reuse

Reusing the same generatedParserResult ends up in strange panics:
See prometheus#7131 and prometheus#7127.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

promql/parser/parse.go

@@ -169,6 +169,7 @@ func newParser(input string) *parser {
 
 	p.injecting = false
 	p.parseErrors = nil
+	p.generatedParserResult = nil
 
 	// Clear lexer struct before reusing.
 	p.lex = Lexer{