fix(v2): initialize modS before usage

fixes db-migrate#665

Signed-off-by: Tobias Gurtzick <[email protected]>

Author: wzrdtales

lib/learn.js

@@ -217,6 +217,8 @@ class STD {
   changeColumn (t, c, s) {
     this.checkColumn(t, c);
 
+    if (!this.modS[t]) this.modS[t] = {};
+
     this.modS[t][c] = this.schema[t][c];
     this.schema[t][c] = Object.assign(this.schema[t][c], s);