fix BN_hex2bn()/BN_dec2bn() memory leak

Reviewed-by: NKurt Roeckx <kurt@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>

crypto/bn/bn_print.c

@@ -139,7 +139,7 @@ int BN_hex2bn(BIGNUM **bn, const char *a)
     for (i = 0; i <= (INT_MAX/4) && isxdigit((unsigned char)a[i]); i++)
         continue;
 
-    if (i > INT_MAX/4)
+    if (i == 0 || i > INT_MAX/4)
         goto err;
 
     num = i + neg;
@@ -209,7 +209,7 @@ int BN_dec2bn(BIGNUM **bn, const char *a)
     for (i = 0; i <= (INT_MAX/4) && isdigit((unsigned char)a[i]); i++)
         continue;
 
-    if (i > INT_MAX/4)
+    if (i == 0 || i > INT_MAX/4)
         goto err;
 
     num = i + neg;