[libxml2] Fix unwanted warnings when switching encodings

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 3aca7f31cb9901dc3af449e08dda647898bfc1fe
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Mon Aug 21 13:06:29 2017 +0200

    Fix unwanted warnings when switching encodings
    
    Revert part of commit 46dc989 "Don't switch encoding for internal
    parameter entities" that caused spurious warnings.
    
    Fixes bug 786267.

 parserInternals.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/parserInternals.c b/parserInternals.c
index d98227c..ef9e56b 100644
--- a/parserInternals.c
+++ b/parserInternals.c
@@ -1235,7 +1235,11 @@ xmlSwitchInputEncodingInt(xmlParserCtxtPtr ctxt, xmlParserInputPtr input,
            input->buf->rawconsumed += use - xmlBufUse(input->buf->raw);
         }
         return (0);
-    } else {
+    } else if (input->length == 0) {
+       /*
+        * When parsing a static memory array one must know the
+        * size to be able to convert the buffer.
+        */
        xmlErrInternal(ctxt, "switching encoding : no input\n", NULL);
        return (-1);
     }