imp: fix segfault on missing user

Problem: if a compute node cannot resolve the target userid,
the imp segfaults due to an unchecked error condition:

  #0  0x000000556a7a7680 imp_exec_privileged (flux-imp + 0x7680)
  flux-framework/flux-core#1  0x000000556a7a3b3c imp_parent (flux-imp + 0x3b3c)
  flux-framework/flux-core#2  0x0000007fac65fe18 __libc_start_main (libc.so.6 + 0x20e18)
  flux-framework/flux-core#3  0x000000556a7a3c18 $x (flux-imp + 0x3c18)
  flux-framework/flux-core#4  0x000000556a7a3c18 $x (flux-imp + 0x3c18)

Check for userid lookup failure and print a helpful message, then die.

Fixes flux-framework#170

src/imp/exec/exec.c

@@ -306,6 +306,14 @@ int imp_exec_privileged (struct imp_state *imp, struct kv *kv)
     if (imp_supports_pam (exec)) {
 #if HAVE_PAM
         struct passwd *user_pwd = passwd_from_uid (exec->userid);
+        if (!user_pwd) {
+            char hostname[1024] = "unknown";
+            (void)gethostname (hostname, sizeof (hostname));
+            imp_die (1,
+                     "exec: userid %d is invalid on %s",
+                     (int)exec->userid,
+                     hostname);
+        }
         if (pam_setup (user_pwd->pw_name) < 0)
             imp_die (1, "exec: PAM stack failure");
 #else