Fix crash in detectPlatform: handle negative e_lfanew offset

For pure DOS executables, the e_lfanew field at MZ header offset 0x3C
is uninitialized garbage. Duke Nukem 2's EXE had bytes that form a
negative int32 (-1878619136), which was used as an array index causing
ArrayIndexOutOfBoundsException.

Now checks peOffset < 0 and returns 'dos' (no Windows signature found).

src/main/java/com/dostalgia/UploadResource.java

@@ -468,14 +468,17 @@ public class UploadResource {
             // Must start with MZ
             if (buf[0] != 0x4D || buf[1] != 0x5A) return null;
 
-            // Read e_lfanew at offset 0x3C
+            // Read e_lfanew at offset 0x3C (only meaningful for PE/NE — garbage for pure DOS)
             if (read < 0x40) return "dos"; // too short for PE/NE, assume DOS
             int peOffset = (buf[0x3C] & 0xFF)
                          | ((buf[0x3D] & 0xFF) << 8)
                          | ((buf[0x3E] & 0xFF) << 16)
                          | ((buf[0x3F] & 0xFF) << 24);
 
-            // If signature is beyond our buffer, read it
+            // e_lfanew is garbage for pure DOS executables — may be negative or absurd
+            if (peOffset < 0) return "dos";
+
+            // If signature is beyond our buffer, read it from the file
             byte sig1, sig2;
             if (peOffset + 2 <= read) {
                 sig1 = buf[peOffset];