#3248: Fixes critical assertion when null string is passed to guarded_markup_escape_text().
#3248: Fixes critical assertion when null string is passed to guarded_markup_escape_text().


jof/shotwell,jimbonator/shotwell,jof/shotwell,damianmoore/shotwell,polymeris/shotwell,ritchiewilson/shotwell,abrauchli/shotwell,damianmoore/shotwell,h4ck3rm1k3/shotwell,h4ck3rm1k3/shotwell,polymeris/shotwell,Sanva/shotwell-gsoc,abrauchli/shotwell,ritchiewilson/shotwell,Sanva/shotwell-gsoc,jimbonator/shotwell,jimbonator/shotwell,Sanva/shotwell-gsoc,damianmoore/shotwell,jof/shotwell,polymeris/shotwell,h4ck3rm1k3/shotwell,abrauchli/shotwell,ritchiewilson/shotwell