(str_append): Check for NULL.

@@ -132,6 +132,8 @@ static int str_append_n(char** to, const char* from, size_t n)
 }
 static int str_append(char** to, const char* from)
 {
+  if (!from)
+    return 0;
   return str_append_n(to, from, strlen(from));
 }
 static int str_append_char(char** to, char c)