Fix off by 1 in ASN1_STRING_set()

Reviewed-by: Rich Salz <rsalz@openssl.org>

MR: #3176
(cherry picked from commit a73be79)

crypto/asn1/asn1_lib.c

@@ -370,7 +370,7 @@ int ASN1_STRING_set(ASN1_STRING *str, const void *_data, int len)
         else
             len = strlen(data);
     }
-    if ((str->length < len) || (str->data == NULL)) {
+    if ((str->length <= len) || (str->data == NULL)) {
         c = str->data;
         if (c == NULL)
             str->data = OPENSSL_malloc(len + 1);