fix a potential kernel crash

add a regression test for it

user/usertests.c

@@ -1908,6 +1908,18 @@ stacktest(char *s)
     exit(xstatus);
 }
 
+// copyinstr() used to cast the virtual page address to uint,
+// which (with certain wild system call arguments) could
+// result in a kernel page fault.
+void
+pgbug(char *s)
+{
+  char *argv[1];
+  argv[0] = 0;
+  exec((char*)0xeaeb0b5b00002f5e, argv);
+  exit(0);
+}
+
 // run each test in its own process. run returns 1 if child's exit()
 // indicates success.
 int
@@ -1945,6 +1957,7 @@ main(int argc, char *argv[])
     void (*f)(char *);
     char *s;
   } tests[] = {
+    {pgbug, "pgbug" },
     {reparent, "reparent" },
     {twochildren, "twochildren"},
     {forkfork, "forkfork"},