Much easier to fix the problem one level deeper. Just replace /bin/bash with this Perl script:

#!/usr/bin/perl -wT
use strict;

# Clean up environment 
s/^\(\) {.*// for values %ENV;

# Now, exec bash with our name and our arguments
exec { $0 } '/bin/bash.original', @ARGV;
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If you're looking at validating all environment variables, you'd need to know which environment variables are supposed to hold what kind of values. And for example LD_PRELOAD or LD_LIBRARY_PATH should be passed through verbatim (because if an attacker already has access to these, you can't even trust yourself).