fix fromBytes for zero length arrays

stew/byteutils.nim

@@ -145,8 +145,9 @@ func toBytes*(s: string): seq[byte] =
   @(s.toOpenArrayByte(0, s.high))
 
 func fromBytes*(T: type string, v: openArray[byte]): string =
-  result = newString(v.len)
-  copyMem(addr result[0], unsafeAddr v[0], v.len)
+  if v.len > 0:
+    result = newString(v.len)
+    copyMem(addr result[0], unsafeAddr v[0], v.len)
 
 func `<`*(a, b: openArray[byte]): bool =
   ## Lexicographical compare of two byte arrays

tests/test_byteutils.nim

@@ -88,6 +88,9 @@ suite "Byte utils":
       string.fromBytes([byte(ord('a'))]) == "a"
       cast[ptr UncheckedArray[byte]](cstring(string.fromBytes([byte(ord('a'))])))[1] == byte(0)
 
+      "".toBytes().len() == 0
+      string.fromBytes([]) == ""
+
   test "slices":
     var a: array[4, byte]
     a[0..<2] = [2'u8, 3]