Fix SIGSEGV when substituting sequence with shorter one

src/Task.cpp

@@ -1393,7 +1393,7 @@ void Task::substitute (
       int skew = 0;
       for (unsigned int i = 0; i < start.size () && !done; ++i)
       {
-        description.replace (start[i + skew], end[i] - start[i], to);
+        description.replace (start[i] + skew, end[i] - start[i], to);
         skew += to.length () - (end[i] - start[i]);
         ++changes;
 

test/substitute.t

@@ -84,6 +84,14 @@ class TestSubstitutions(TestCase):
         code, out, err = self.t("_get 1.description")
         self.assertEqual("aaa BbB\n", out)
 
+    def test_substitution_long_with_short(self):
+        """Verify substitution of a sequence with a shorter sequence."""
+        self.t("add aaaaBaaaa")
+        self.t("1 modify /aaaa/c/g")
+        code, out, err = self.t("_get 1.description")
+        self.assertEqual("cBc\n", out)
+
+
 class TestBug441(TestCase):
     def setUp(self):
         self.t = Task()