fix bug where XD would segfault if the cwd didn't exist

also include errno so we can check it

XD.c

@@ -3,7 +3,10 @@
 #include <string.h>
 #include <ctype.h>
 
-#if defined(ERR) || defined(EXPLAIN)
+#ifdef ERR
+#include <errno.h>
+#endif
+#if defined(EXPLAIN) || defined(ERR)
 #include <stdarg.h>
 #endif
 
@@ -62,9 +65,12 @@ char
   struct stat s;
   int i, c;
 
-
   /* find the number of jumps to the root of the fs */
   rpath = realpath(path, NULL);
+  if (!rpath) {
+    L("realpath: %s", strerror(errno));
+    return NULL;
+  }
   for (i = c = 0; i < strlen(rpath); i++) {
     if (rpath[i] == '/') {
       c++;