Simplify the SRAM's representation in NDSCartArgs (#1914)

* Simplify the SRAM's representation in  `NDSCartArgs`

- I overthought this one.
- I could've just checked `args && args->SRAM`, but then some other poor bastard might make this mistake.
- Don't mix `pair`, `optional`, and `unique_ptr` all at once, kids.

* Fix a `nullptr` read

src/NDSCart.cpp

@@ -404,7 +404,11 @@ CartRetail::CartRetail(std::unique_ptr<u8[]>&& rom, u32 len, u32 chipid, bool ba
         { // Copy in what we can, truncate the rest.
             SRAM = std::make_unique<u8[]>(SRAMLength);
             memset(SRAM.get(), 0xFF, SRAMLength);
-            memcpy(SRAM.get(), sram.get(), std::min(sramlen, SRAMLength));
+
+            if (sram)
+            { // If we have anything to copy, that is.
+                memcpy(SRAM.get(), sram.get(), std::min(sramlen, SRAMLength));
+            }
         }
     }
 
@@ -1650,7 +1654,8 @@ std::unique_ptr<CartCommon> ParseROM(std::unique_ptr<u8[]>&& romdata, u32 romlen
     }
 
     std::unique_ptr<CartCommon> cart;
-    auto [sram, sramlen] = args ? std::move(*args->SRAM) : std::make_pair(nullptr, 0);
+    std::unique_ptr<u8[]> sram = args ? std::move(args->SRAM) : nullptr;
+    u32 sramlen = args ? args->SRAMLength : 0;
     if (homebrew)
         cart = std::make_unique<CartHomebrew>(std::move(cartrom), cartromsize, cartid, romparams, args ? std::move(args->SDCard) : std::nullopt);
     else if (gametitle[0] == 0 && !strncmp("SD/TF-NDS", gametitle + 1, 9) && gamecode == 0x414D5341)