Address issues found by coverity

1) `mbedtls_rsa_import_raw` used an uninitialized return
   value when it was called without any input parameters.
   While not sensible, this is allowed and should be a
   succeeding no-op.

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

3) Both the RSA keygeneration example program and the
   RSA test suites 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 of the respective function orphaned.
   While this defect existed before, Coverity picked up on
   it again because of newly introduced MPI's that were
   also wrongly initialized only after the call to CTR DRBG
   init. The commit fixes both the old and the new issue
   by moving the initializtion of both the RSA context and
   all MPI's prior to the first potentially failing call.

library/rsa.c

@@ -104,7 +104,7 @@ int mbedtls_rsa_import_raw( mbedtls_rsa_context *ctx,
                             unsigned char const *D, size_t D_len,
                             unsigned char const *E, size_t E_len )
 {
-    int ret;
+    int ret = 0;
 
     if( N != NULL )
     {