nshlib/nsh_consolemain: detect null pointer

Returning with an error from nsh_consolemain prevents dereferencing a
NULL pointer later in the code.

nshlib/nsh_consolemain.c

@@ -65,6 +65,10 @@ int nsh_consolemain(int argc, FAR char *argv[])
   int ret;
 
   DEBUGASSERT(pstate != NULL);
+  if (pstate == NULL)
+    {
+      return -ENOMEM;
+    }
 
   /* Execute the session */