Fix mxmlSaveString with a 0-length buffer (Issue #284)

CHANGES.md

@@ -1,6 +1,7 @@
 # Changes in Mini-XML 3.2.1
 
 - Fixed potential memory leak in `mxmlLoad*` functions (Issue #278, Issue #279)
+- Fixed `mxmlSaveString` with a buffer size of 0 (Issue #284)
 - Fixed `MXML_MINOR_VERSION` value in "mxml.h" (Issue #285)
 - Fixed POSIX threading support for MingW (Issue #287)
 - Fixed some minor memory leaks found by Coverity.

mxml-file.c

@@ -376,7 +376,10 @@ mxmlSaveString(mxml_node_t    *node,	/* I - Node to write */
   */
 
   if (ptr[0] >= ptr[1])
-    buffer[bufsize - 1] = '\0';
+  {
+    if (bufsize > 0)
+      buffer[bufsize - 1] = '\0';
+  }
   else
     ptr[0][0] = '\0';