Followup fix for mantis 3049 (from Yarn)

Revert an Error back to a Warning
Prevent an out-of-bounds read of the Lcl_ext_str array

git-svn-id: svn://svn.icculus.org/fs2open@11008 387891d4-d844-0410-90c0-e4c51a9137d3
Fix mantis 3097 (valgrind)

Don't read past the start of the string

git-svn-id: svn://svn.icculus.org/fs2open@11009 387891d4-d844-0410-90c0-e4c51a9137d3