Fix hmac_drbg failure in benchmark, with threading

Remove redunadnat calls to `hmac_drbg_free()` between seeding operations,
which make the mutex invalid. Fixes #1095
diff --git a/programs/test/benchmark.c b/programs/test/benchmark.c
index 5277ceb..e7d29c3 100644
--- a/programs/test/benchmark.c
+++ b/programs/test/benchmark.c
@@ -700,7 +700,6 @@
             mbedtls_exit(1);
         TIME_AND_TSC( "HMAC_DRBG SHA-1 (NOPR)",
                 mbedtls_hmac_drbg_random( &hmac_drbg, buf, BUFSIZE ) );
-        mbedtls_hmac_drbg_free( &hmac_drbg );
 
         if( mbedtls_hmac_drbg_seed( &hmac_drbg, md_info, myrand, NULL, NULL, 0 ) != 0 )
             mbedtls_exit(1);
@@ -708,7 +707,6 @@
                                              MBEDTLS_HMAC_DRBG_PR_ON );
         TIME_AND_TSC( "HMAC_DRBG SHA-1 (PR)",
                 mbedtls_hmac_drbg_random( &hmac_drbg, buf, BUFSIZE ) );
-        mbedtls_hmac_drbg_free( &hmac_drbg );
 #endif
 
 #if defined(MBEDTLS_SHA256_C)
@@ -719,7 +717,6 @@
             mbedtls_exit(1);
         TIME_AND_TSC( "HMAC_DRBG SHA-256 (NOPR)",
                 mbedtls_hmac_drbg_random( &hmac_drbg, buf, BUFSIZE ) );
-        mbedtls_hmac_drbg_free( &hmac_drbg );
 
         if( mbedtls_hmac_drbg_seed( &hmac_drbg, md_info, myrand, NULL, NULL, 0 ) != 0 )
             mbedtls_exit(1);
@@ -727,8 +724,8 @@
                                              MBEDTLS_HMAC_DRBG_PR_ON );
         TIME_AND_TSC( "HMAC_DRBG SHA-256 (PR)",
                 mbedtls_hmac_drbg_random( &hmac_drbg, buf, BUFSIZE ) );
-        mbedtls_hmac_drbg_free( &hmac_drbg );
 #endif
+        mbedtls_hmac_drbg_free( &hmac_drbg );
     }
 #endif