Fix some buffer conversion issues

https://bugzilla.gnome.org/show_bug.cgi?id=690202

Buffer overflow errors originating from xmlBufGetInputBase in 2.9.0
The pointers from the context input were not properly reset after
that call which can do reallocations.
diff --git a/HTMLparser.c b/HTMLparser.c
index a533f37..6b83654 100644
--- a/HTMLparser.c
+++ b/HTMLparser.c
@@ -6054,6 +6054,8 @@
 	    if ((in->encoder != NULL) && (in->buffer != NULL) &&
 		    (in->raw != NULL)) {
 		int nbchars;
+		size_t base = xmlBufGetInputBase(in->buffer, ctxt->input);
+		size_t current = ctxt->input->cur - ctxt->input->base;
 
 		nbchars = xmlCharEncInput(in);
 		if (nbchars < 0) {
@@ -6061,6 +6063,7 @@
 			         "encoder error\n", NULL, NULL);
 		    return(XML_ERR_INVALID_ENCODING);
 		}
+		xmlBufSetInputBaseCur(in->buffer, ctxt->input, base, current);
 	    }
 	}
     }