strip minor improvement (#16444)

* strip minor improvement
* add more tests
* Update tests/stdlib/tstrutils.nim

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

lib/pure/strutils.nim

@@ -2813,7 +2813,7 @@ func strip*(s: string, leading = true, trailing = true,
   if leading:
     while first <= last and s[first] in chars: inc(first)
   if trailing:
-    while last >= 0 and s[last] in chars: dec(last)
+    while last >= first and s[last] in chars: dec(last)
   result = substr(s, first, last)
 
 func stripLineEnd*(s: var string) =

tests/stdlib/tstrutils.nim

@@ -28,6 +28,19 @@ template main() =
     doAssert strip("sfoofoofoos", leading = false, chars = {'s'}) == "sfoofoofoo"
     doAssert strip("sfoofoofoos", trailing = false, chars = {'s'}) == "foofoofoos"
 
+    block:
+      let a = "xxxxxx"
+      doAssert a.strip(chars={'x'}).len == 0
+
+    doAssert "".strip(chars={'x'}).len == 0
+    doAssert "         ".strip(chars={'x'}) == "         "
+    doAssert "xxx xxx".strip(chars={'x'}) == " "
+    doAssert "xxx  wind".strip(chars={'x'}) == "  wind"
+    doAssert "xxx  iii".strip(chars={'i'}) == "xxx  "
+    doAssert "x".strip(leading = false, chars={'x'}).len == 0
+    doAssert "x".strip(trailing = false, chars={'x'}).len == 0
+    doAssert "x".strip(leading = false, trailing = false, chars={'x'}) == "x"
+
   block: # split
     var ret: seq[string] # or use `toSeq` or `collect`
     for p in split("/home/a1:xyz:/usr/bin", {':'}): ret.add p