make tmemfile2 work again

compiler/ccgexprs.nim

@@ -1769,6 +1769,7 @@ proc convStrToCStr(p: BProc, n: PNode, d: var TLoc) =
   initLocExpr(p, n.sons[0], a)
   putIntoDest(p, d, n,
               ropecg(p.module, "#nimToCStringConv($1)", [rdLoc(a)]),
+#                "($1 ? $1->data : (NCSTRING)\"\")" % [a.rdLoc],
               a.storage)
 
 proc convCStrToStr(p: BProc, n: PNode, d: var TLoc) =

lib/pure/memfiles.nim

@@ -399,7 +399,8 @@ iterator lines*(mfile: MemFile, buf: var TaintedString, delim='\l', eat='\r'): T
 
   for ms in memSlices(mfile, delim, eat):
     setLen(buf.string, ms.size)
-    copyMem(buf.cstring, ms.data, ms.size)
+    if ms.size > 0:
+      copyMem(addr buf[0], ms.data, ms.size)
     yield buf
 
 iterator lines*(mfile: MemFile, delim='\l', eat='\r'): TaintedString {.inline.} =

tests/stdlib/tmemfiles2.nim

@@ -18,8 +18,9 @@ mm.close()
 
 # read, change
 mm_full = memfiles.open(fn, mode = fmWrite, mappedSize = -1, allowRemap = true)
-echo "Full read size: ",mm_full.size
+let size = mm_full.size
 p = mm_full.mapMem(fmReadWrite, 20, 0)
+echo "Full read size: ", size
 var p2 = cast[cstring](p)
 p2[0] = 'H'
 p2[1] = 'e'