newruntime: fixes another bug

lib/core/seqs.nim

@@ -104,7 +104,7 @@ proc newSeqPayload(cap, elemSize: int): pointer {.compilerRtl, raises: [].} =
 proc prepareSeqAdd(len: int; p: pointer; addlen, elemSize: int): pointer {.
     compilerRtl, noSideEffect, raises: [].} =
   {.noSideEffect.}:
-    if len+addlen <= len:
+    if addlen <= 0:
       result = p
     elif p == nil:
       result = newSeqPayload(len+addlen, elemSize)

lib/core/strs.nim

@@ -171,8 +171,7 @@ proc setLengthStrV2(s: var NimStringV2, newLen: int) {.compilerRtl.} =
   s.len = newLen
 
 proc nimAsgnStrV2(a: var NimStringV2, b: NimStringV2) {.compilerRtl.} =
-  # self assignment is fine!
-  #if unlikely(a.p == b.p): return
+  if a.p == b.p: return
   if isLiteral(b):
     # we can shallow copy literals:
     frees(a)