Fix the basic_static_cstring array ctor to respect the no-embedded-NULs invariant

The constructor taking a const CharT (&)[M] copied M - 1 characters
verbatim via assign(arr, M - 1), preserving any embedded NULs in the
input. This would violate the type's documented "no embedded NULs"
invariant.

  const char arr[] = { '1', '2', '\0', '4', '5', '\0' };
  static_cstring<5> s(arr);
  // data_ holds {'1','2','\0','4','5','\0'} --- embedded NUL stored

Delegate to assign(arr) instead, which uses traits::length() to
determine the size. This matches the behavior of the other
CharT-accepting entry points (assign(const CharT*), the const CharT*
constructor) and makes the invariant self-enforcing at construction.

Note: In practice, this is a bug that would never surface, because the
array constructor is never chosen for static_cstring<N> s(arr)---the
pointer constructor always wins---but the array constructor is needed
for CTAD (deduction guide), so it better be correct.

This also means the test we added is, at the moment, superfluous.

Author: gennaroprota

example/static_cstring/static_cstring.hpp

@@ -123,7 +123,7 @@ class basic_static_cstring
     constexpr basic_static_cstring(const CharT (&arr)[M])
     {
         static_assert(M <= N + 1, "Array too big for static_cstring");
-        assign(arr, M - 1);
+        assign(arr);
     }
 
     constexpr size_type size() const noexcept

example/static_cstring/static_cstring_test.cpp

@@ -154,6 +154,13 @@ testCStringConstruct()
         BOOST_TEST(*s1.end() == 0);
     }
 
+    // Construct from a C string with embedded NULs.
+    {
+        const char arr[] = { '1', '2', '\0', '4', '5', '\0' };
+        static_cstring<5> s1(arr);
+        BOOST_TEST(s1 == "12" );
+    }
+
     // Copy construction.
     {
         static_cstring<5> s1("12345");