bn/bn_mod.c: harmonize BN_mod_add_quick with original implementation.

New implementation failed to correctly reset r->neg flag. Spotted by
OSSFuzz.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6783)

crypto/bn/bn_mod.c

@@ -83,6 +83,7 @@ int bn_mod_add_fixed_top(BIGNUM *r, const BIGNUM *a, const BIGNUM *b,
         ((volatile BN_ULONG *)tp)[i] = 0;
     }
     r->top = mtop;
+    r->neg = 0;
 
     if (tp != storage)
         OPENSSL_free(tp);