Fix StackOverflow in ThrowableProxy

[huguesb]

It is possible for the cause of a suppressed exception to be the exception it is attached to. Gracefully handle that case by unsetting the cause to break and avoid a StackOverflow.

Sample stack trace showing the issue:

javax.net.ssl.SSLException: SSL peer shut down incorrectly
        at sun.security.ssl.InputRecord.readV3Record(InputRecord.java:596)
        at sun.security.ssl.InputRecord.read(InputRecord.java:532)
        at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:961)
        at sun.security.ssl.SSLSocketImpl.readDataRecord(SSLSocketImpl.java:918)
        at sun.security.ssl.AppInputStream.read(AppInputStream.java:105)
        at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
        at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
        at sun.net.www.MeteredStream.read(MeteredStream.java:134)
        at java.io.FilterInputStream.read(FilterInputStream.java:133)
        at sun.net.www.protocol.http.HttpURLConnection.read(HttpURLConnection.java:3335)
        at sun.net.www.protocol.http.HttpURLConnection.read(HttpURLConnection.java:3328)
        ...
        Suppressed: javax.net.ssl.SSLException: Connection has been shutdown: javax.net.ssl.SSLException: SSL peer shut down incorrectly
                at sun.security.ssl.SSLSocketImpl.checkEOF(SSLSocketImpl.java:1529)
                at sun.security.ssl.AppInputStream.available(AppInputStream.java:60)
                at java.io.BufferedInputStream.available(BufferedInputStream.java:410)
                at sun.net.www.MeteredStream.available(MeteredStream.java:170)
                at sun.net.www.http.KeepAliveStream.close(KeepAliveStream.java:85)
                at java.io.FilterInputStream.close(FilterInputStream.java:181)
                at sun.net.www.protocol.http.HttpURLConnection.close(HttpURLConnection.java:3408)
                ...