Die on -D=foo, when parsing options - we can't let someone define (nil).

MFC after:	3 days

usr.bin/m4/main.c

@@ -204,6 +204,8 @@ main(int argc, char *argv[])
 			for (p = optarg; *p; p++)
 				if (*p == '=')
 					break;
+			if (p == optarg)
+				errx(1, "null variable cannot be defined");
 			if (*p)
 				*p++ = EOS;
 			dodefine(optarg, p);