glcpp: Fix post-decrement underflow in loop-control variable

glcpp: Fix post-decrement underflow in loop-control variable

This loop-control condition with a post-decrement operator would lead to
an underflow of collapsed_newlines. This in turn would cause a subsequent
execution of the loop to labor inordinately trying to return the loop-control
variable to a value of 0 again.

Fix this by dis-intertwining the test and the decrement.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65112

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>