encoding/xml: reject processing instructions with reserved names

This is required by the spec. Fixes: #68499

Author: DemiMarie

src/encoding/xml/xml.go

@@ -610,6 +610,11 @@ func (d *Decoder) rawToken() (Token, error) {
 }
 return nil, d.err
 }
+if len(target) >= 3 && target[0:3] != xmlPrefix &&
+ (target[0] | 0x20) == 'x' && (target[1] | 0x20) == 'm' && (target[2] | 0x20) == 'l' {
+d.err = d.syntaxError("Processing instruction name is reserved")
+return nil, d.err
+}
 d.space()
 d.buf.Reset()
 var b0 byte