Downgrade to setlen when not (nimSeqsV2 or hasAlloc)

Without `hasAlloc` `system/sysstr` is not included in system
so `setLengthStrUninit` is not in the scope.

Author: ZoomRmc

lib/system.nim

@@ -2320,7 +2320,10 @@ func setLenUninit*(s: var string, newlen: Natural) {.nodestroy.} =
           setLengthStrV2Uninit(cast[ptr NimStringV2](str)[], newlen)
       else:
         {.noSideEffect.}:
-          setLengthStrUninit(s, newlen)
+          when hasAlloc:
+            setLengthStrUninit(s, newlen)
+          else:
+            s.setLen(n)
     else: s.setLen(n)
 
 

lib/system/strs_v2.nim

@@ -166,7 +166,7 @@ proc setLengthStrV2(s: var NimStringV2, newLen: int) {.compilerRtl.} =
     s.p.data[newLen] = '\0'
   s.len = newLen
 
-proc setLengthStrV2Uninit(s: var NimStringV2, newLen: int)  =
+proc setLengthStrV2Uninit(s: var NimStringV2, newLen: int) =
   if newLen == 0:
     discard "do not free the buffer here, pattern 's.setLen 0' is common for avoiding allocations"
   else: