dpkg: Do not segfault on “dpkg -i --no-act”

mkdtemp() takes the string allocated by path_make_temp_template() and
modifies it, but there's not enough allocated room for the subsequent
string appends done starting from cidirrest, for the control files.

Regression introduced in commit ffccc65580189420a0a64736bba0fb661de56dcb.

debian/changelog

@@ -31,6 +31,7 @@ dpkg (1.16.0) UNRELEASED; urgency=low
   * By default reject installing packages w/o an Architecture field. They
     now need --force-architecture, dpkg will still warn about them though.
   * Fix build failure when passing --disable-nls to configure.
+  * Do not segfault on “dpkg -i --no-act”.
 
   [ Raphaël Hertzog ]
   * Fail properly when debian/source/format is empty. Closes: #600854

src/processarc.c

@@ -219,14 +219,21 @@ void process_archive(const char *filename) {
     deb_verify(filename);
 
   if (f_noact) {
+    char *tmpdir;
+
     if (!cidirbuf)
       free(cidirbuf);
-    cidir = cidirbuf = mkdtemp(path_make_temp_template("dpkg"));
-    if (!cidir)
+    tmpdir = mkdtemp(path_make_temp_template("dpkg"));
+    if (!tmpdir)
       ohshite(_("unable to create temporary directory"));
+
+    cidir = cidirbuf = m_malloc(strlen(tmpdir) + MAXCONTROLFILENAME + 10);
+    strcpy(cidir, tmpdir);
     strcat(cidir,"/");
 
     cidirrest = cidir + strlen(cidir);
+
+    free(tmpdir);
   } else {
     /* We want it to be on the same filesystem so that we can
      * use rename(2) to install the postinst &c. */