Fix few warnings resulting in build failures

library/hmac_drbg.c

@@ -74,7 +74,7 @@ int mbedtls_hmac_drbg_update_ret( mbedtls_hmac_drbg_context *ctx,
     unsigned char rounds = ( additional != NULL && add_len != 0 ) ? 2 : 1;
     unsigned char sep[1];
     unsigned char K[MBEDTLS_MD_MAX_SIZE];
-    int ret;
+    int ret = 0;
 
     for( sep[0] = 0; sep[0] < rounds; sep[0]++ )
     {

library/x509_crt.c

@@ -2013,7 +2013,7 @@ static int x509_crt_find_parent_in(
 {
     int ret;
     mbedtls_x509_crt *parent, *fallback_parent;
-    int signature_is_good, fallback_signature_is_good;
+    int signature_is_good = 0, fallback_signature_is_good;
 
 #if defined(MBEDTLS_ECDSA_C) && defined(MBEDTLS_ECP_RESTARTABLE)
     /* did we have something in progress? */