dpkg-query: Fix strtol() errno check when parsing the COLUMNS envvar

Regression introduced in commit 3d258742dfe5cd18e4e06a5fbd855b99bb95046e.

Closes: #827265
Signed-off-by: Guillem Jover <guillem@debian.org>

debian/changelog

@@ -25,6 +25,8 @@ dpkg (1.18.8) UNRELEASED; urgency=medium
   * Activate file triggers for conffiles on purge, which has never happened
     before. Before dpkg 1.17.0, conffiles were triggered on removal, which
     was obviously wrong. Reported by Helmut Grohne <helmut@subdivi.de>.
+  * Fix strtol() errno check when parsing the COLUMNS envvar in dpkg-query.
+    Thanks to Sven Joachim <svenjoac@gmx.de>. Closes: #827265
   * Perl modules:
     - Use warnings::warnif() instead of carp() for deprecated warnings.
     - Add new format_range() method and deprecate dpkg() and rfc822() methods

src/querycmd.c

@@ -74,7 +74,7 @@ static int getwidth(void) {
   if (columns) {
     errno = 0;
     res = strtol(columns, &endptr, 10);
-    if (errno != 0 && columns != endptr && *endptr == '\0' &&
+    if (errno == 0 && columns != endptr && *endptr == '\0' &&
         res > 0 && res < INT_MAX)
       return res;
   }