fix: add null checks for std::get_if<EncodableMap> call sites

[aki1770-del]

Problem

std::get_if<EncodableMap>() returns nullptr when the Flutter method call argument is not an EncodableMap (e.g., wrong type or missing arguments). All affected call sites dereference the pointer unconditionally with *args, which is undefined behavior and causes an IVI HMI crash.

Fixes: #226

Changes

Added an explicit null check with result->Error() before every *args dereference across 6 plugins:

Plugin	File	Call sites fixed
secure_storage	messages.cc	1
camera	messages.cc	6
audioplayers_linux	messages.cc	1
nav_render_view	nav_render_texture.cc	1
pdf	messages.cc	2
desktop_window_linux	messages.cc	7

Total: 18 call sites guarded.

Fix pattern

// Before
const auto& args = std::get_if<EncodableMap>(call.arguments());
for (const auto& [fst, snd] : *args) { ... }  // UB if args == nullptr

// After
const auto& args = std::get_if<EncodableMap>(call.arguments());
if (!args) {
  result->Error("argument_error", "Expected map arguments");
  return;  // return std::nullopt for desktop_window_linux
}
for (const auto& [fst, snd] : *args) { ... }

Verification

• Zero deletions — existing logic is unchanged
• Each guard returns a typed error instead of crashing
• desktop_window_linux uses return std::nullopt to match the surrounding lambda return type

Signed-off-by: Akihiro Komada aki1770@gmail.com