fix(xml): panic when encoding a sequence tagged as a map

The pre-pass that emits processing instructions first walks Content in
key/value pairs, which is only valid for a mapping, but it gated on
node.Tag instead of node.Kind. A sequence tagged !!map can have an odd
Content length, so Content[i+1] ran off the end:

    printf '!!map [cat, dog, fish]\n' | yq -o=xml
    panic: runtime error: index out of range [3] with length 3

That pre-pass runs before the switch on node.Kind, which would otherwise
have returned the usual 'cannot encode ... to XML' error.

The same check also skipped well formed mappings that carry a custom
tag, so their processing instructions were dropped: encodeTopLevelMap
then skipped the key as already processed and it disappeared from the
output altogether.

Gate the pre-pass on Kind, as the rest of the file already does in the
switch in Encode, in doEncode and in encodeMap.

pkg/yqlib/encoder_xml.go

@@ -46,7 +46,7 @@ func (e *xmlEncoder) Encode(writer io.Writer, node *CandidateNode) error {
 	encoder.Indent("", e.indentString)
 	var newLine xml.CharData = []byte("\n")
 
-	if node.Tag == "!!map" {
+	if node.Kind == MappingNode {
 		// make sure <?xml .. ?> processing instructions are encoded first
 		for i := 0; i < len(node.Content); i += 2 {
 			key := node.Content[i]

pkg/yqlib/xml_test.go

@@ -557,6 +557,20 @@ var xmlScenarios = []formatScenario{
 		expectedError: "cannot encode !!seq to XML - only maps can be encoded",
 		scenarioType:  "encode-error",
 	},
+	{
+		description:   "arrays tagged as maps cannot be encoded",
+		skipDoc:       true,
+		input:         "!!map [cat, dog, fish]",
+		expectedError: "cannot encode !!map to XML - only maps can be encoded",
+		scenarioType:  "encode-error",
+	},
+	{
+		description:  "proc instructions of a map with a custom tag",
+		skipDoc:      true,
+		input:        "!custom\n+p_xml: version=\"1.0\"\na: b\n",
+		expected:     "<?xml version=\"1.0\"?>\n<a>b</a>\n",
+		scenarioType: "encode",
+	},
 	{
 		description:    "Encode xml: attributes with content",
 		subdescription: "Fields with the matching xml-content-name is assumed to be content.",