xmlGrowArray: guard against NULL capacity pointer

xmlmemory.c

diff --git a/xmlmemory.c b/xmlmemory.c
index 90306e3..0875a79 100644
--- a/xmlmemory.c
+++ b/xmlmemory.c
@@ -367,6 +367,9 @@
     void *tmp;
     int newSize;
 
+    if (capacity == NULL)
+        return(NULL);
+
     newSize = xmlGrowCapacity(*capacity, elemSize, min, max);
     if (newSize < 0)
         return(NULL);