fix #H7079 - melting ice destroys lowered dbridge

melt_ice() attempted to handle this but checked for raised drawbridge
instead of lowered one.

doc/fixes36.1

@@ -541,6 +541,7 @@ at the prompt for entering a level annotation, responding with <return>
 	erroneously removed old annotation; use <space><return> to do that
 reorder the #droptype menu and add some separator lines
 moving from a pit to an adjacent pit with uncleared debris shouldn't be a fall
+open drawbridge spanning ice would vanish if the ice melted
 
 
 Fixes to Post-3.6.0 Problems that Were Exposed Via git Repository