Backport 2.7:Add guard to out_left to avoid negative values

Add guard to out_left to avoid negative values
diff --git a/ChangeLog b/ChangeLog
index 09bb3cb..2deaafb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -33,6 +33,8 @@
    * Fix typo in a comment ctr_drbg.c. Contributed by Paul Sokolovsky.
    * MD functions deprecated in 2.7.0 are no longer inline, to provide
      a migration path for those depending on the library's ABI.
+   * Add guard to validate that out_left can not be negative. Raised by 
+     samoconnor in #1245.
 
 = mbed TLS 2.7.0 branch released 2018-02-03