Allow zero sized memory input buffers

Useful for a fuzz target I'm working on.
diff --git a/buf.c b/buf.c
index 07922ff..3b76018 100644
--- a/buf.c
+++ b/buf.c
@@ -231,7 +231,7 @@
 xmlBufCreateStatic(void *mem, size_t size) {
     xmlBufPtr ret;
 
-    if ((mem == NULL) || (size == 0))
+    if (mem == NULL)
         return(NULL);
 
     ret = (xmlBufPtr) xmlMalloc(sizeof(xmlBuf));