Fix CVE-2020-10595:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10595

Patch copied from upstream advisory:

https://seclists.org/oss-sec/2020/q1/128

diff --git a/prompting.c b/prompting.c
index e985d95..d81054f 100644
--- a/prompting.c
+++ b/prompting.c
@@ -314,26 +314,27 @@ pamk5_prompter_krb5(krb5_context context UNUSED, void *data, const char *name,
     /*
      * Reuse pam_prompts as a starting index and copy the data into the reply
      * area of the krb5_prompt structs.
      */
     pam_prompts = 0;
     if (name != NULL && !args->silent)
         pam_prompts++;
     if (banner != NULL && !args->silent)
         pam_prompts++;
     for (i = 0; i < num_prompts; i++, pam_prompts++) {
-        size_t len;
+        size_t len, allowed;

         if (resp[pam_prompts].resp == NULL)
             goto cleanup;
         len = strlen(resp[pam_prompts].resp);
-        if (len > prompts[i].reply->length)
+        allowed = prompts[i].reply->length;
+        if (allowed == 0 || len > allowed - 1)
             goto cleanup;

         /*
          * The trailing nul is not included in length, but other applications
          * expect it to be there.  Therefore, we copy one more byte than the
          * actual length of the password, but set length to just the length of
          * the password.
          */
         memcpy(prompts[i].reply->data, resp[pam_prompts].resp, len + 1);
         prompts[i].reply->length = (unsigned int) len;
od='get' action='/guix/log/HACKING'>
<input type='hidden' name='id' value='d9de8fbbab2143e6d576ed379e0b35c6d1955714'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=d9de8fbbab2143e6d576ed379e0b35c6d1955714'>root</a>/<a href='/guix/log/HACKING?id=d9de8fbbab2143e6d576ed379e0b35c6d1955714'>HACKING</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/HACKING?id=d9de8fbbab2143e6d576ed379e0b35c6d1955714&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-07-13 14:25:19 -0400'>2019-07-13</span></td><td><a href='/guix/commit/HACKING?id=06e1ecbf17fee7fe513ad2808d8175fb3565ae3e'>doc: Update keyserver URL.</a><span class='msg-avail'>...<span class='msg-tooltip'>* HACKING (Commit Access): Change keyserver URL to 'keys.openpgp.org'.
</span></span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2019-02-11 23:23:27 +0100'>2019-02-11</span></td><td><a href='/guix/commit/HACKING?id=6701edbfad62ba9572ffd35497df3db27b208ba5'>HACKING: Update key server.</a><span class='msg-avail'>...<span class='msg-tooltip'>* HACKING (Commit Access): Recommend pool.sks-keyservers.net.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-07-22 17:00:47 +0200'>2018-07-22</span></td><td><a href='/guix/commit/HACKING?id=5285192150c83db12136af6a123e03bb9b9dbe5f'>doc: Fix typo (missing period)</a><span class='msg-avail'>...<span class='msg-tooltip'>* HACKING (Commit Access): Fix typo.
</span></span></td><td>Pierre Neidhardt</td></tr>
<tr><td><span title='2017-08-19 13:42:52 +0530'>2017-08-19</span></td><td><a href='/guix/commit/HACKING?id=acabbd597678e1f0be995297bb009b6dd38ae795'>doc: Mention how to use emacs-debbugs.</a><span class='msg-avail'>...<span class='msg-tooltip'>* HACKING (Using emacs-debbugs): Add section on using emacs-debbugs.
</span></span></td><td>Arun Isaac</td></tr>