fixes #17768 [backport:1.4] (#18317)

* fixes #17768 [backport:1.4]

* tiny

(cherry picked from commit 2deb7011f5)

lib/pure/unicode.nim

@@ -333,7 +333,7 @@ proc runeReverseOffset*(s: string, rev: Positive): (int, int) =
   ## from the end (starting with 1) and the total
   ## number of runes in the string.
   ##
-  ## Returns a negative value for offset if there are to few runes in
+  ## Returns a negative value for offset if there are too few runes in
   ## the string to satisfy the request.
   ##
   ## **Beware:** This can lead to unoptimized code and slow execution!
@@ -346,16 +346,14 @@ proc runeReverseOffset*(s: string, rev: Positive): (int, int) =
     a = rev.int
     o = 0
     x = 0
+  let times = 2*rev.int-s.runeLen # transformed from rev.int - a < s.runeLen - rev.int
   while o < s.len:
     let r = runeLenAt(s, o)
     o += r
-    if a < 0:
+    if a > times:
       x += r
     dec a
-
-  if a > 0:
-    return (-a, rev.int-a)
-  return (x, -a+rev.int)
+  result = if a > 0: (-a, rev.int-a) else: (x, -a+rev.int)
 
 proc runeAtPos*(s: string, pos: int): Rune =
   ## Returns the rune at position ``pos``.