Handle "set height 1"

PR cli/17151 points out that "set height 1" has pathological behavior
in gdb.  What I see is that gdb will endlessly print the pagination
prompt.  This patch takes a simple and expedient approach to a fix:
pretend that the height is really 2.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17151