lib/rsa: correct check after allocation in fdt_add_bignum()

After allocating to pointer ctx we should check that pointer and not
another pointer already checked above.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/rsa/rsa-sign.c b/lib/rsa/rsa-sign.c
index 40ca1e1..1f0d81b 100644
--- a/lib/rsa/rsa-sign.c
+++ b/lib/rsa/rsa-sign.c
@@ -708,7 +708,7 @@
 		return -ENOMEM;
 	}
 	ctx = BN_CTX_new();
-	if (!tmp) {
+	if (!ctx) {
 		fprintf(stderr, "Out of memory (bignum context)\n");
 		return -ENOMEM;
 	}