[libxml2] Allow zero sized memory input buffers

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 94f6ce838c6a903ef5d0251f061a3af1816b4c50
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Thu Jun 8 22:36:09 2017 +0200

    Allow zero sized memory input buffers
    
    Useful for a fuzz target I'm working on.

 buf.c   |    2 +-
 xmlIO.c |    4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/buf.c b/buf.c
index 07922ff..3b76018 100644
--- a/buf.c
+++ b/buf.c
@@ -231,7 +231,7 @@ xmlBufPtr
 xmlBufCreateStatic(void *mem, size_t size) {
     xmlBufPtr ret;
 
-    if ((mem == NULL) || (size == 0))
+    if (mem == NULL)
         return(NULL);
 
     ret = (xmlBufPtr) xmlMalloc(sizeof(xmlBuf));
diff --git a/xmlIO.c b/xmlIO.c
index 300ee47..8c6509d 100644
--- a/xmlIO.c
+++ b/xmlIO.c
@@ -3027,7 +3027,7 @@ xmlParserInputBufferCreateMem(const char *mem, int size, xmlCharEncoding enc) {
     xmlParserInputBufferPtr ret;
     int errcode;
 
-    if (size <= 0) return(NULL);
+    if (size < 0) return(NULL);
     if (mem == NULL) return(NULL);
 
     ret = xmlAllocParserInputBuffer(enc);
@@ -3063,7 +3063,7 @@ xmlParserInputBufferCreateStatic(const char *mem, int size,
                                  xmlCharEncoding enc) {
     xmlParserInputBufferPtr ret;
 
-    if (size <= 0) return(NULL);
+    if (size < 0) return(NULL);
     if (mem == NULL) return(NULL);
 
     ret = (xmlParserInputBufferPtr) xmlMalloc(sizeof(xmlParserInputBuffer));