Don't forget to free G, P, Q, ctr_drbg, and entropy

I might be wrong, but lcc's optimizer is curious about this,
and I am too: shouldn't we free allocated stuff correctly
before exiting `dh_genprime` in this certain point of code?

Signed-off-by: makise-homura <akemi_homura@kurisa.ch>

programs/pkey/dh_genprime.c

@@ -91,7 +91,7 @@ int main( int argc, char **argv )
     {
     usage:
         mbedtls_printf( USAGE );
-        mbedtls_exit( exit_code );
+        goto exit;
     }
 
     for( i = 1; i < argc; i++ )