fixes #12965 (#12991)

lib/system/strs_v2.nim

@@ -37,7 +37,8 @@ proc resize(old: int): int {.inline.} =
   else: result = old * 3 div 2 # for large arrays * 3/2 is better
 
 proc prepareAdd(s: var NimStringV2; addlen: int) {.compilerRtl.} =
-  if isLiteral(s) and addlen > 0:
+  if isLiteral(s):
+    if addlen > 0:
       let oldP = s.p
       # can't mutate a literal, so we need a fresh copy here:
       let allocator = getLocalAllocator()

tests/destructor/tnewruntime_strutils.nim

@@ -210,5 +210,9 @@ proc staticTests =
 nonStaticTests()
 staticTests()
 
+# bug #12965
+let xaa = @[""].join()
+let xbb = @["", ""].join()
+
 let (a, d) = allocCounters()
 discard cprintf("%ld %ld\n", a, d)