Cost limiting not enforced in C extensions. #27

I noticed that using a very large cost with BCrypt::Password.create would not increase the runtime on MRI. When I tested this under JRuby, the Java Extensions would raise a "bad number of rounds" ArgumentError if the cost was over 31. The C extensions should have the same behavior as the Java Extensions.

Steps to Reproduce

require 'bcrypt'
require 'benchmark'

# base
puts Benchmark.measure { BCrypt::Password.create("hello", :cost => 10) }

# midpoint
puts Benchmark.measure { BCrypt::Password.create("hello", :cost => 32) }

# extreme
puts Benchmark.measure { BCrypt::Password.create("hello", :cost => (2 ** 30)) }

Actual Results

MRI

Base:
0.090000 0.000000 0.090000 ( 0.094195)

Midpoint:
+5 minutes

Extreme:
0.000000 0.000000 0.000000 ( 0.001570)

JRuby

Base:
0.295000 0.000000 0.295000 ( 0.296000)

Midpoint:
NativeException: java.lang.IllegalArgumentException: Bad number of rounds

Extreme:
NativeException: java.lang.IllegalArgumentException: Missing salt rounds