util.termwidth: never return 0 for terminal width

Catch a case where the termwidth was being reported as 0 when I was connecting
with TLMTask instead of NSTask in OSX. This caused the progress extension to
print no progress. The termwidth should never return 0 so in case we would
return 0, simply fall back to the default termwidth below which is 80.

mercurial/posix.py

@@ -316,7 +316,9 b' def termwidth():'
                 if not os.isatty(fd):
                     continue
                 arri = fcntl.ioctl(fd, termios.TIOCGWINSZ, '\0' * 8)
-                return array.array('h', arri)[1]
+                width = array.array('h', arri)[1]
+                if width > 0:
+                    return width
             except ValueError:
                 pass
             except IOError, e: