Fix #14994 (#14996)

* Fix #14994

* Revert misplaced "optimization"

* Typo

lib/pure/collections/sequtils.nim

@@ -527,7 +527,8 @@ proc insert*[T](dest: var seq[T], src: openArray[T], pos = 0) =
     assert dest == outcome
 
   var j = len(dest) - 1
-  var i = len(dest) + len(src) - 1
+  var i = j + len(src)
+  if i == j: return
   dest.setLen(i + 1)
 
   # Move items after `pos` to the end of the sequence.

lib/system/seqs_v2.nim

@@ -82,6 +82,7 @@ proc shrink*[T](x: var seq[T]; newLen: Natural) =
 
 proc grow*[T](x: var seq[T]; newLen: Natural; value: T) =
   let oldLen = x.len
+  #sysAssert newLen >= x.len, "invalid newLen parameter for 'grow'"
   if newLen <= oldLen: return
   var xu = cast[ptr NimSeqV2[T]](addr x)
   if xu.p == nil or xu.p.cap < newLen: