Ensure that rngfill returns the requested amount of randomness:

One of the two versions of the `rngfill` function accepts a pointer
to a buffer and a size (in bytes). The function aims to fill the
provided `buffer` with `size` random bytes. It does this in chunks
of 8 bytes, for long as possible, and then fills any left-over gap
one byte at a time.

To avoid an annoying and incorrect warning about a potential buffer
overflow in the "trailing write", commit 78bc2727f7
used a `#pragma` to instruct the compiler to not generate the incorrect
diagnostic. Unfortunately, this change _also_ eliminated the trailing
write code, which means that, under some cases, the `rngfill` function
would generate between 1 and 7 fewer random bytes than requested.

This problem would only manifest on builds that do not define `__GNUC__`
which, as of this writing, means MSVC.

src/ripple/beast/utility/rngfill.h

@@ -32,6 +32,7 @@ void
 rngfill(void* buffer, std::size_t bytes, Generator& g)
 {
     using result_type = typename Generator::result_type;
+
     while (bytes >= sizeof(result_type))
     {
         auto const v = g();
@@ -39,15 +40,22 @@ rngfill(void* buffer, std::size_t bytes, Generator& g)
         buffer = reinterpret_cast<std::uint8_t*>(buffer) + sizeof(v);
         bytes -= sizeof(v);
     }
+
+    assert(bytes < sizeof(result_type));
+
 #ifdef __GNUC__
     // gcc 11.1 (falsely) warns about an array-bounds overflow in release mode.
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Warray-bounds"
+#endif
+
     if (bytes > 0)
     {
         auto const v = g();
         std::memcpy(buffer, &v, bytes);
     }
+
+#ifdef __GNUC__
 #pragma GCC diagnostic pop
 #endif
 }