Add entry describing the bug fix
Add entry describing the bug fix in `mbedtls_ecdsa_genkey()`.
mbed TLS ChangeLog (Sorted per branch, date)
+= mbed TLS x.xx.x branch released xxxx-xx-xx
+
+Bugfix
+ * Fix returning the value 1 when mbedtls_ecdsa_genkey failed.
+
= mbed TLS 2.16.0 branch released 2018-12-21
Features
