RFR 8182999: SunEC throws ProviderException on invalid curves

[Adam Petcher]

This is a bug fix related to invalid curves in the SunEC provider. 
During ECKeyPairGenerator.initialize(), the provider only checks whether 
the curve is known, but it doesn't check whether the curve is actually 
supported by the native code. So the call to generateKeyPair() can fail 
in the native code and throw a ProviderException. This change adds a new 
native method to check whether the curve is supported. This method is 
called by initialize(), which will set the state to uninitialized and 
throw the expected exception when the curve is not supported.

JBS: https://bugs.openjdk.java.net/browse/JDK-8182999
Webrev: http://cr.openjdk.java.net/~apetcher/8182999/webrev.00/