Fix crash when trying to open unreadable IWAD files

src/common/utility/cmdlib.cpp

@@ -165,6 +165,23 @@ bool FileExists (const char *filename)
 	return res && !isdir;
 }
 
+//==========================================================================
+//
+// FileReadable
+//
+// Returns true if the file can be read.
+//
+//==========================================================================
+
+bool FileReadable(const char *filename)
+{
+#ifndef _WIN32
+	return access (filename, R_OK) == 0;
+#else
+	return _access (filename, 4) == 0;
+#endif
+}
+
 //==========================================================================
 //
 // DirExists

src/common/utility/cmdlib.h

@@ -33,6 +33,7 @@ char(&_ArraySizeHelper(T(&array)[N]))[N];
 #define myoffsetof(type,identifier) ((size_t)&((type *)alignof(type))->identifier - alignof(type))
 
 bool FileExists (const char *filename);
+bool FileReadable (const char *filename);
 bool DirExists(const char *filename);
 bool DirEntryExists (const char *pathname, bool *isdir = nullptr);
 bool GetFileInfo(const char* pathname, size_t* size, time_t* time);

src/d_iwad.cpp

@@ -577,10 +577,10 @@ int FIWadManager::IdentifyVersion (TArray<FString> &wadfiles, const char *iwad,
 	// -iwad not found or not specified. Revert back to standard behavior.
 	if (mFoundWads.Size() == numFoundWads) iwadparm = nullptr;
 
-	// Check for symbolic links leading to non-existent files.
+	// Check for symbolic links leading to non-existent files and for files that are unreadable.
 	for (unsigned int i = 0; i < mFoundWads.Size(); i++)
 	{
-		if (!FileExists(mFoundWads[i].mFullPath)) mFoundWads.Delete(i);
+		if (!FileExists(mFoundWads[i].mFullPath) || !FileReadable(mFoundWads[i].mFullPath)) mFoundWads.Delete(i);
 	}
 
 	// Now check if what got collected actually is an IWAD.