t/loc_tools.pl: Add checks

This verifies that we can restore a locale that we were previously in,
and makes sure that we don't stay in a locale that doesn't work well.
Doing so has led to crashes.

t/loc_tools.pl

@@ -178,6 +178,10 @@ ($$$$)
 
         no locale;
 
+        # We definitely don't want the locale set to something that is
+        # unsupported
+        die "Couldn't restore locale '$save_locale', category $category"
+                                      unless setlocale($category, $save_locale);
         if ($badutf8) {
             _my_fail("Verify locale name doesn't contain malformed utf8");
             return;
@@ -642,7 +646,9 @@ (;$)
         setlocale(&POSIX::LC_CTYPE(), $locale);
         push @return, $locale if uc('i') eq "\x{130}";
     }
-    setlocale(&POSIX::LC_CTYPE(), $save_locale);
+
+    die "Couldn't restore locale '$save_locale'"
+                            unless setlocale(&POSIX::LC_CTYPE(), $save_locale);
 
     return @return;
 }