fix: prevent null issues when strings are unequal

In the case where s2 is larger than s1, this errors because the index is
out of range.

apheleia-rcs.el

@@ -46,10 +46,11 @@ text of S1 surrounding P1."
          (i1 (length s1))
          (i2 (length s2)))
     (while (> i1 p1)
-      (let ((ins (1+ (gethash (cons i1 (1- i2)) table)))
-            (del (1+ (gethash (cons (1- i1) i2) table)))
-            (sub (gethash (cons (1- i1) (1- i2)) table)))
-        (unless (= (aref s1 (1- i1)) (aref s2 (1- i2)))
+      (let ((ins (1+ (or (gethash (cons i1 (1- i2)) table) 1)))
+            (del (1+ (or (gethash (cons (1- i1) i2) table) 1)))
+            (sub (or (gethash (cons (1- i1) (1- i2)) table) 1)))
+        (unless (and (> 0 i2)
+                     (= (aref s1 (1- i1)) (aref s2 (1- i2))))
           (cl-incf sub))
         (let ((cost (min ins del sub)))
           (cond