[LyX/master] Avoid an assertion.

[Enrico Forestieri]

commit 061509bf756193d1bc0a754e0cf8dab62869ea9d
Author: Enrico Forestieri <for...@lyx.org>
Date:   Wed May 28 20:50:45 2014 +0200

    Avoid an assertion.
    
    Do not assert if an inset separator is the only item of a List
    environment. Although it is a weird thing to do, both GUI and
    latex output can deal with it.

diff --git a/src/TextMetrics.cpp b/src/TextMetrics.cpp
index e8053cb..1b434a5 100644
--- a/src/TextMetrics.cpp
+++ b/src/TextMetrics.cpp
@@ -695,13 +695,13 @@ int TextMetrics::labelFill(pit_type const pit, Row const 
& row) const
        Paragraph const & par = text_->getPar(pit);
 
        pos_type last = par.beginOfBody();
-       LBUFERR(last > 0);
+       LBUFERR(last > 0 || par.isEnvSeparator(0));
 
        // -1 because a label ends with a space that is in the label
        --last;
 
        // a separator at this end does not count
-       if (par.isLineSeparator(last))
+       if (last >= 0 && par.isLineSeparator(last))
                --last;
 
        int w = 0;