check null pointer before using memcpy()

Author: ygj6

include/msgpack/sbuffer.h

@@ -59,6 +59,7 @@ static inline void msgpack_sbuffer_free(msgpack_sbuffer* sbuf)
 static inline int msgpack_sbuffer_write(void* data, const char* buf, size_t len)
 {
     msgpack_sbuffer* sbuf = (msgpack_sbuffer*)data;
+    assert(buf || len == 0);
 
     if(sbuf->alloc - sbuf->size < len) {
         void* tmp;
@@ -81,8 +82,10 @@ static inline int msgpack_sbuffer_write(void* data, const char* buf, size_t len)
         sbuf->alloc = nsize;
     }
 
-    memcpy(sbuf->data + sbuf->size, buf, len);
-    sbuf->size += len;
+    if(buf) {
+        memcpy(sbuf->data + sbuf->size, buf, len);
+        sbuf->size += len;
+    }
     return 0;
 }