Fix crash when just -d is given

This would result in a pointer dereference of one past the end of argv.
Add some tests to check this and similar conditions.

Makefile

@@ -92,12 +92,20 @@ little unknown:
 # This can be used to run a program with something like wine or qemu
 TESTWRAPPER=
 test:
+	$(TESTWRAPPER) ./fvcbm --
+	$(TESTWRAPPER) ./fvcbm -d --
+	$(TESTWRAPPER) ./fvcbm || test "$$?" = 1
+	$(TESTWRAPPER) ./fvcbm > generate.txt || test "$$?" = 1
+	$(TESTWRAPPER) ./fvcbm -h > generate.txt || test "$$?" = 1
+	$(TESTWRAPPER) ./fvcbm -d > generate.txt || test "$$?" = 1
 	$(TESTWRAPPER) ./fvcbm testdata/* > generate.txt 2>&1
 	diff expect.txt generate.txt
 	$(TESTWRAPPER) ./fvcbm -d testdata/* > generate.txt 2>&1
 	diff expect-d.txt generate.txt
 	$(TESTWRAPPER) ./fvcbm testdata/test1 > generate.txt 2>&1
 	diff expect-x.txt generate.txt
+	$(TESTWRAPPER) ./fvcbm -- testdata/test1 > generate.txt 2>&1
+	diff expect-x.txt generate.txt
 
 #
 # fvcbm targets below this line

fvcbm.c

@@ -227,7 +227,20 @@ int main(int argc, char *argv[])
 	setvbuf(stdout, NULL, _IOLBF, 82);		/* speed up screen output */
 #endif
 
-	if ((argc < 2) ||
+	if ((argc > 1) &&
+		((argv[FirstFileName][0] == '-') || (argv[FirstFileName][0] == '/')) &&
+		((argv[FirstFileName][1] == 'd')
+#ifdef CPM
+		  || (argv[FirstFileName][1] == 'D')
+#endif
+		) && (argv[FirstFileName][2] == '\x0')) {
+		WideFormat = 0;		/* 1541-style output */
+		++FirstFileName;
+	} else {
+		WideFormat = 1;		/* wide FV-style output */
+	}
+
+	if ((argc <= FirstFileName) ||
 		(((argv[FirstFileName][0] == '-') || (argv[FirstFileName][0] == '/')) &&
 		 ((argv[FirstFileName][1] == '?') || (argv[FirstFileName][1] == 'h')
 #ifdef CPM
@@ -246,18 +259,6 @@ int main(int argc, char *argv[])
 		return 1;
 	}
 
-	if (((argv[FirstFileName][0] == '-') || (argv[FirstFileName][0] == '/')) &&
-		((argv[FirstFileName][1] == 'd')
-#ifdef CPM
-		  || (argv[FirstFileName][1] == 'D')
-#endif
-		) && (argv[FirstFileName][2] == '\x0')) {
-		WideFormat = 0;		/* 1541-style output */
-		++FirstFileName;
-	} else {
-		WideFormat = 1;		/* wide FV-style output */
-	}
-
 	/* -- ends options */
 	if ((argv[FirstFileName][0] == '-') && (argv[FirstFileName][1] == '-')) {
 		++FirstFileName;