Address issues found by coverity

1) The MPI test for prime generation missed a return value
   check for a call to `mpi_shift_r`. This is neither
   critical nor new but should be fixed.

2) The RSA keygeneration example program contained code
   initializing an RSA context after a potentially failing
   call to CTR DRBG initialization, leaving the corresponding
   RSA context free call in the cleanup section orphaned.
   The commit fixes this by moving the initializtion of the
   RSA context prior to the first potentially failing call.
diff --git a/programs/pkey/rsa_genkey.c b/programs/pkey/rsa_genkey.c
index 0270b53..305158b 100644
--- a/programs/pkey/rsa_genkey.c
+++ b/programs/pkey/rsa_genkey.c
@@ -74,6 +74,7 @@
     fflush( stdout );
 
     entropy_init( &entropy );
+    rsa_init( &rsa, RSA_PKCS_V15, 0 );
     if( ( ret = ctr_drbg_init( &ctr_drbg, entropy_func, &entropy,
                                (const unsigned char *) pers,
                                strlen( pers ) ) ) != 0 )
@@ -85,7 +86,6 @@
     polarssl_printf( " ok\n  . Generating the RSA key [ %d-bit ]...", KEY_SIZE );
     fflush( stdout );
 
-    rsa_init( &rsa, RSA_PKCS_V15, 0 );
 
     if( ( ret = rsa_gen_key( &rsa, ctr_drbg_random, &ctr_drbg, KEY_SIZE,
                              EXPONENT ) ) != 0 )