Fix integer overflow in cheatsImportGSACodeFile length check.

Although a length check is being performed on the imported GSA Codes file, `len` is both a signed int and attacker controlled.

With a specially crafted GSA Codes file, an attacker could specify a value for `len` that overflows the `int` type, rolling over into a negative number. By doing so, the attacker can bypass the conditional mentioned above.

The `fseek` length parameter is of type `size_t` which is an unsigned int, this will result in `len` being interpreted as a large unsigned int, allowing for a stack based buffed overflow in the desc char array.

By making `len` an unsigned integer, it will prevent the overflow. It ensures that the bounds check works as intended.

src/gba/Cheats.cpp

@@ -2047,7 +2047,7 @@ bool cheatsImportGSACodeFile(const char* name, int game, bool v3)
         return false;
     }
 
-    int len = 0;
+    uint32_t len = 0;
     bool found = false;
     int g = 0;
     while (games > 0) {