fix for issue 1118: check if iv is zero in gcm.

1) found by roberto in mbedtls forum
2) if iv_len is zero, return an error
3) add tests for invalid parameters

ChangeLog

@@ -1,6 +1,10 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
-= mbed TLS 2.x.x released xxxx-xx-xx
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+   * Add a check if iv_len is zero, and return an error if it is zero. reported
+     by roberto. #716
 
 Features
    * Add the functions mbedtls_platform_setup() and mbedtls_platform_teardown()