script/execute: Don't let trailing blank lines determine the return code

The grub_script_execute_sourcecode() parses and executes code one line
at a time, updating the return code each time because only the last line
determines the final status. However, trailing new lines were also
executed, masking any failure on the previous line. Fix this by only
trying to execute the command when there is actually one present.

This has presumably never been noticed because this code is not used by
regular functions, only in special cases like eval and menu entries. The
latter generally don't return at all, having booted an OS. When failing
to boot, upstream GRUB triggers the fallback mechanism regardless of the
return code.

We noticed the problem while using Red Hat's patches, which change this
behaviour to take account of the return code. In that case, a failure
takes you back to the menu rather than triggering a fallback.

Signed-off-by: James Le Cuirot <jlecuirot@microsoft.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/script/execute.c

@@ -914,7 +914,10 @@ grub_script_execute_sourcecode (const char *source)
 	  break;
 	}
 
+      /* Don't let trailing blank lines determine the return code. */
+      if (parsed_script->cmd)
 	ret = grub_script_execute (parsed_script);
+
       grub_script_free (parsed_script);
       grub_free (line);
     }

tests/grub_script_eval.in

@@ -4,3 +4,11 @@ eval echo "Hello world"
 valname=tst
 eval $valname=hi
 echo $tst
+
+if eval "
+false
+"; then
+  echo should have failed
+else
+  echo failed as expected
+fi