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.
diff --git a/library/pkcs5.c b/library/pkcs5.c
index 8e55415..9826c5d 100644
--- a/library/pkcs5.c
+++ b/library/pkcs5.c
@@ -407,7 +407,7 @@
 
     printf( "\n" );
 
-    return( 0 );
+    return( ret );
 }
 
 #endif /* POLARSSL_SELF_TEST */