Prefer initializing ret to error values

These initial values shouldn't be used, but in case they accidentally
get used after a code change, fail safe.
diff --git a/library/entropy.c b/library/entropy.c
index ac7e905..d7091cb 100644
--- a/library/entropy.c
+++ b/library/entropy.c
@@ -258,7 +258,9 @@
  */
 static int entropy_gather_internal( mbedtls_entropy_context *ctx )
 {
-    int ret = 0, i, have_one_strong = 0;
+    int ret = MBEDTLS_ERR_ENTROPY_SOURCE_FAILED;
+    int i;
+    int have_one_strong = 0;
     unsigned char buf[MBEDTLS_ENTROPY_MAX_GATHER];
     size_t olen;