Bug 733824 - PDF export fails without warning if open in other application.

This happens on the Windows platforms in particular where applications
can lock files, preventing GIMP to obtain a file descriptor for writing.

plug-ins/common/file-pdf-save.c

@@ -418,6 +418,11 @@ run (const gchar      *name,
     }
 
   fp = g_fopen (file_name, "wb");
+  if (fp == NULL)
+    {
+      values[0].data.d_status = GIMP_PDB_EXECUTION_ERROR;
+      return;
+    }
 
   pdf_file = cairo_pdf_surface_create_for_stream (write_func, fp, 1, 1);