Make sure slice is not out of range when reading XML prolog

http/client/response.go

@@ -84,7 +84,12 @@ func (r *Response) EnsureUnicodeBody() (err error) {
 
 			// We ignore documents with encoding specified in XML prolog.
 			// This is going to be handled by the XML parser.
-			if xmlEncodingRegex.Match(buffer[0:1024]) {
+			length := 1024
+			if len(buffer) < 1024 {
+				length = len(buffer)
+			}
+
+			if xmlEncodingRegex.Match(buffer[0:length]) {
 				return
 			}
 		}

http/client/response_test.go

@@ -123,6 +123,7 @@ func TestEnsureUnicodeWithHTMLDocuments(t *testing.T) {
 		{"rdf_utf8.xml", "application/rss+xml; charset=utf-8", true},
 		{"charset-content-type-xml-iso88591.xml", "application/rss+xml; charset=ISO-8859-1", false},
 		{"windows_1251.xml", "text/xml", false},
+		{"smallfile.xml", "text/xml; charset=utf-8", true},
 	}
 
 	for _, tc := range unicodeTestCases {

http/client/testdata/smallfile.xml

@@ -0,0 +1 @@
+<?xml version="1.0"?>