Add position check for XML declaration

## 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: ruby#161 (comment)

lib/rexml/parsers/baseparser.rb

@@ -635,21 +635,25 @@ 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"
-          content = match_data[2]
-          version = VERSION.match(content)
-          version = version[1] unless version.nil?
-          encoding = ENCODING.match(content)
-          encoding = encoding[1] unless encoding.nil?
-          if need_source_encoding_update?(encoding)
-            @source.encoding = encoding
-          end
-          if encoding.nil? and /\AUTF-16(?:BE|LE)\z/i =~ @source.encoding
-            encoding = "UTF-16"
+        if match_data[1] == "xml"
+          if @document_status
+            raise ParseException.new("Malformed XML: XML declaration other than at the top of the document.", @source)
+          else
+            content = match_data[2]
+            version = VERSION.match(content)
+            version = version[1] unless version.nil?
+            encoding = ENCODING.match(content)
+            encoding = encoding[1] unless encoding.nil?
+            if need_source_encoding_update?(encoding)
+              @source.encoding = encoding
+            end
+            if encoding.nil? and /\AUTF-16(?:BE|LE)\z/i =~ @source.encoding
+              encoding = "UTF-16"
+            end
+            standalone = STANDALONE.match(content)
+            standalone = standalone[1] unless standalone.nil?
+            return [ :xmldecl, version, encoding, standalone ]
           end
-          standalone = STANDALONE.match(content)
-          standalone = standalone[1] unless standalone.nil?
-          return [ :xmldecl, version, encoding, standalone ]
         end
         [:processing_instruction, match_data[1], match_data[2]]
       end

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 other than at the top of the document.
+          Line: 1
+          Position: 25
+          Last 80 unconsumed characters:
+
+        DETAIL
+      end
     end
   end
 end