Minor fixes in POP3 analyzer based on flycheck warnings

This also adds an implementation of unreachable() to use for default
cases where it shouldn't ever actually cause the default.

src/util.h

@@ -661,5 +661,20 @@ inline std::vector<std::wstring_view> split(const wchar_t* s, const wchar_t* del
     return split(std::wstring_view(s), std::wstring_view(delim));
 }
 
+/**
+ * Implementation of std::unreachable. Once C++23 is supported this can be replaced with
+ * an alias. This implementation is taken from cppreference.
+ */
+[[noreturn]] inline void unreachable() {
+    // Uses compiler specific extensions if possible.  Even if no extension is used,
+    // undefined behavior is still raised by an empty function body and the noreturn
+    // attribute.
+#if defined(_MSC_VER) && ! defined(__clang__) // MSVC
+    __assume(false);
+#else // GCC, Clang
+    __builtin_unreachable();
+#endif
+}
+
 } // namespace util
 } // namespace zeek