pkcs5.c: fix dead store: return proper exit status

Found with Clang's `scan-build` tool.

The error value assigned to `ret` is not returned, meaning that the
selftest always succeeds. Ensure the error value is propagated back to
the caller.

library/pkcs5.c

@@ -408,7 +408,7 @@ int pkcs5_self_test( int verbose )
 exit:
     md_free( &sha1_ctx );
 
-    return( 0 );
+    return( ret );
 }
 #endif /* POLARSSL_SHA1_C */