follow up #22380; fixes incorrect usages of newWideCString (#23278)

follow up #22380

lib/pure/os.nim

@@ -424,13 +424,13 @@ proc expandFilename*(filename: string): string {.rtl, extern: "nos$1",
   when defined(windows):
     var bufsize = MAX_PATH.int32
     var unused: WideCString = nil
-    var res = newWideCString("", bufsize)
+    var res = newWideCString(bufsize)
     while true:
       var L = getFullPathNameW(newWideCString(filename), bufsize, res, unused)
       if L == 0'i32:
         raiseOSError(osLastError(), filename)
       elif L > bufsize:
-        res = newWideCString("", L)
+        res = newWideCString(L)
         bufsize = L
       else:
         result = res$L

lib/std/private/ospaths2.nim

@@ -862,13 +862,13 @@ when not defined(nimscript):
       raiseAssert "use -d:nodejs to have `getCurrentDir` defined"
     elif defined(windows):
       var bufsize = MAX_PATH.int32
-      var res = newWideCString("", bufsize)
+      var res = newWideCString(bufsize)
       while true:
         var L = getCurrentDirectoryW(bufsize, res)
         if L == 0'i32:
           raiseOSError(osLastError())
         elif L > bufsize:
-          res = newWideCString("", L)
+          res = newWideCString(L)
           bufsize = L
         else:
           result = res$L

lib/std/syncio.nim

@@ -419,7 +419,7 @@ proc readLine*(f: File, line: var string): bool {.tags: [ReadIOEffect],
     if f.isatty:
       const numberOfCharsToRead = 2048
       var numberOfCharsRead = 0'i32
-      var buffer = newWideCString("", numberOfCharsToRead)
+      var buffer = newWideCString(numberOfCharsToRead)
       if readConsole(getOsFileHandle(f), addr(buffer[0]),
         numberOfCharsToRead, addr(numberOfCharsRead), nil) == 0:
         var error = getLastError()

lib/std/widestrs.nim

@@ -154,6 +154,7 @@ when not (defined(cpu16) or defined(cpu8)):
     createWide(result, size * 2 + 2)
 
   proc newWideCString*(source: cstring, L: int): WideCStringObj =
+    ## Warning:: `source` needs to be preallocated with the length `L`
     createWide(result, L * 2 + 2)
     var d = 0
     for ch in runes(source, L):