fix ropes format errors (#16106) [backport:1.0]

* fix rope index

* add testcase

* fix ropes format

(cherry picked from commit 0c6c4be0e7)

lib/pure/ropes.nim

@@ -262,7 +262,7 @@ proc `%`*(frmt: string, args: openArray[Rope]): Rope {.
         while true:
           j = j * 10 + ord(frmt[i]) - ord('0')
           inc(i)
-          if frmt[i] notin {'0'..'9'}: break
+          if i >= frmt.len or frmt[i] notin {'0'..'9'}: break
         add(result, args[j-1])
       of '{':
         inc(i)

tests/stdlib/tropes.nim

@@ -64,3 +64,10 @@ block:
     inc i
 
   doAssert r[66] == '\0'
+
+block:
+  let r1 = "$1 $2 $3" % [rope("Nim"), rope("is"), rope("a great language")]
+  doAssert $r1 == "Nim is a great language"
+
+  let r2 = "$# $# $#" % [rope("Nim"), rope("is"), rope("a great language")]
+  doAssert $r2 == "Nim is a great language"