Don't behold and and un-behold the player in the same instant if a mermaid

is behind a transparent wall, but instead give a message about being unable to hear the mermaid.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6215 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on June 29, 2008
23S5SPIJC5VOGKB5RQ3VEEXPUERPYT6WIR54FUSCRENQNGDHUF7AC
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