Fix searchExtPos so that it returns -1 when the path is not a file ext (#22245)

* Fix searchExtPos so that it returns -1 when the path is not a file ext

* fix comparision expression

* Remove splitDrive from searchExtPos

lib/std/private/ospaths2.nim

@@ -584,15 +584,28 @@ proc searchExtPos*(path: string): int =
     assert searchExtPos("c.nim") == 1
     assert searchExtPos("a/b/c.nim") == 5
     assert searchExtPos("a.b.c.nim") == 5
+    assert searchExtPos(".nim") == -1
+    assert searchExtPos("..nim") == -1
+    assert searchExtPos("a..nim") == 2
 
-  # BUGFIX: do not search until 0! .DS_Store is no file extension!
+  # Unless there is any char that is not `ExtSep` before last `ExtSep` in the file name,
+  # it is not a file extension.
+  const DirSeps = when doslikeFileSystem: {DirSep, AltSep, ':'} else: {DirSep, AltSep}
   result = -1
-  for i in countdown(len(path)-1, 1):
+  var i = path.high
+  while i >= 1:
     if path[i] == ExtSep:
+      break
+    elif path[i] in DirSeps:
+      return -1 # do not skip over path
+    dec i
+
+  for j in countdown(i - 1, 0):
+    if path[j] in DirSeps:
+      return -1
+    elif path[j] != ExtSep:
       result = i
       break
-    elif path[i] in {DirSep, AltSep}:
-      break # do not skip over path
 
 proc splitFile*(path: string): tuple[dir, name, ext: string] {.
   noSideEffect, rtl, extern: "nos$1".} =