Fix #12410 (#15685)

* Fix #12410, big thanks to @pmetras for this fix

* Add testcase

lib/pure/streams.nim

@@ -242,7 +242,7 @@ proc readDataStr*(s: Stream, buffer: var string, slice: Slice[int]): int =
     result = s.readDataStrImpl(s, buffer, slice)
   else:
     # fallback
-    result = s.readData(addr buffer[0], buffer.len)
+    result = s.readData(addr buffer[slice.a], slice.b + 1 - slice.a)
 
 template jsOrVmBlock(caseJsOrVm, caseElse: untyped): untyped =
   when nimvm:

tests/stdlib/tstreams.nim

@@ -6,6 +6,7 @@ Nice name: Arne
 fs is: nil
 
 threw exception
+_heh_
 '''
   nimout: '''
 I
@@ -49,3 +50,15 @@ block tstreams3:
     for line in s.lines:
       echo line
     s.close
+
+# bug #12410
+
+var a = newStringStream "hehohihahuhyh"
+a.readDataStrImpl = nil
+
+var buffer = "_ooo_"
+
+doAssert a.readDataStr(buffer, 1..3) == 3
+
+echo buffer
+