Make entropy double-free work

Although the library documentation does not guarantee that calling
mbedtls_entropy_free() twice works, it's a plausible assumption and it's
natural to write code that frees an object twice. While this is uncommon for
an entropy context, which is usually a global variable, it came up in our
own unit tests (random_twice tests in test_suite_random in the
development branch).

Announce this in the same changelog entry as for RSA because it's the same
bug in the two modules.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

include/mbedtls/entropy.h

@@ -147,13 +147,15 @@ mbedtls_entropy_source_state;
  */
 typedef struct mbedtls_entropy_context
 {
-    int accumulator_started;
+    int accumulator_started; /* 0 after init.
+                              * 1 after the first update.
+                              * -1 after free. */
 #if defined(MBEDTLS_ENTROPY_SHA512_ACCUMULATOR)
     mbedtls_sha512_context  accumulator;
 #else
     mbedtls_sha256_context  accumulator;
 #endif
-    int             source_count;
+    int             source_count; /* Number of entries used in source. */
     mbedtls_entropy_source_state    source[MBEDTLS_ENTROPY_MAX_SOURCES];
 #if defined(MBEDTLS_HAVEGE_C)
     mbedtls_havege_state    havege_data;