Fix is_null_terminated reading arbitrary memory (issue #1425) (#1429)

When `_alloc` is equal to `_size`, the `is_null_terminated` method will
point to arbitrary memory when checking for the `0` byte. This PR makes
that method first check that `_alloc != _size` before reading the
`_size` byte of the `Pointer[U8]`.

Fixes #1425

packages/builtin/string.pony

@@ -376,7 +376,7 @@ actor Main
     which may be present earlier in the content of the string.
     If you need a null-terminated copy of this string, use the clone method.
     """
-    (_alloc > 0) and (_ptr._apply(_size) == 0)
+    (_alloc > 0) and (_alloc != _size) and (_ptr._apply(_size) == 0)
 
   fun utf32(offset: ISize): (U32, U8) ? =>
     """

packages/builtin_test/_test.pony

@@ -437,6 +437,13 @@ class iso _TestStringIsNullTerminated is UnitTest
     h.assert_true("0123456".trim(2, 4).clone().is_null_terminated())
     h.assert_false("0123456".trim(2, 4).is_null_terminated())
 
+    h.assert_true(String.from_iso_array(recover
+      ['a', 'b', 'c']
+    end).is_null_terminated())
+    h.assert_false(String.from_iso_array(recover
+      ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h'] // power of two sized array
+    end).is_null_terminated())
+
 
 class iso _TestSpecialValuesF32 is UnitTest
   """