Add position check for XML declaration (#162)

## Why?
XML declaration must be the first item.

https://www.w3.org/TR/2006/REC-xml11-20060816/#document

```
[1]   document   ::=   ( prolog element Misc* ) - ( Char* RestrictedChar Char* )
```

https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-prolog

```
[22]   prolog   ::=   	XMLDecl Misc* (doctypedecl Misc*)?
```

https://www.w3.org/TR/2006/REC-xml11-20060816/#NT-XMLDecl

```
[23]   XMLDecl  ::=   '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'
```

See: #161 (comment)

lib/rexml/parsers/baseparser.rb

@@ -644,7 +644,10 @@ def process_instruction(start_position)
           @source.position = start_position
           raise REXML::ParseException.new(message, @source)
         end
-        if @document_status.nil? and match_data[1] == "xml"
+        if match_data[1] == "xml"
+          if @document_status
+            raise ParseException.new("Malformed XML: XML declaration is not at the start", @source)
+          end
           content = match_data[2]
           version = VERSION.match(content)
           version = version[1] unless version.nil?

test/parse/test_processing_instruction.rb

@@ -39,6 +39,23 @@ def test_garbage_text
                        pi.content,
                      ])
       end
+
+      def test_xml_declaration_not_at_document_start
+        exception = assert_raise(REXML::ParseException) do
+          parser = REXML::Parsers::BaseParser.new('<a><?xml version="1.0" ?></a>')
+          while parser.has_next?
+            parser.pull
+          end
+        end
+
+        assert_equal(<<~DETAIL.chomp, exception.to_s)
+          Malformed XML: XML declaration is not at the start
+          Line: 1
+          Position: 25
+          Last 80 unconsumed characters:
+
+        DETAIL
+      end
     end
 
     def test_after_root