flux-shell: abort if taskmap=block is used with a value

Problem: If --taskmap=block:foo is used, the job shell silently
ignores the value 'foo'. This could lead to a job running with
the wrong taskmap if the user intended some other taskmap scheme.
It would be better to abort and notify the user something's wrong.

Check if a value is specified with `--taskmap=block` and abort the
shell if so.

Author: grondo

src/shell/shell.c

@@ -1337,9 +1337,17 @@ static int shell_taskmap (flux_shell_t *shell)
         shell_log_error ("failed to parse taskmap shell option");
         return -1;
     }
-    if (rc == 0
-        || streq (scheme, "block"))
+    if (rc == 0)
         return 0;
+    if (streq (scheme, "block")) {
+        /*  A value is not allowed for the block taskmap scheme:
+         */
+        if (strlen (value) > 0)
+            shell_die (1,
+                       "block taskmap does not accept a value (got %s)",
+                       value);
+        return 0;
+    }
 
     shell_trace ("remapping tasks with scheme=%s value=%s",
                  scheme,