Avoid segfault when a texture file is missing.

Always check the return value of "findDataFile". If it's empty, always
provide the _original_ path being searched in an error message. Otherwise
we eventually just get useless 'Can't find file "".' error messages
(and also a segfault here, since osgDB::readImageFile("") returns NULL).

simgear/scene/material/mat.cxx

@@ -213,8 +213,14 @@ SGMaterial::read_properties(const SGReaderWriterOptions* options,
         fullMaskPath = SGModelLib::findDataFile(ompath.str(), options);
       }
 
+      if (fullMaskPath.empty()) {
+          SG_LOG(SG_GENERAL, SG_ALERT, "Cannot find texture file \""
+                 << ompath.str() << "\"");
+      }
+      else
+      {
         osg::Image* image = osgDB::readImageFile(fullMaskPath, options);
-      if (image->valid())
+        if (image && image->valid())
         {
           osg::Texture2D* object_mask = new osg::Texture2D;
 
@@ -242,6 +248,7 @@ SGMaterial::read_properties(const SGReaderWriterOptions* options,
         }
       }
     }
+  } 
 
   xsize = props->getDoubleValue("xsize", 0.0);
   ysize = props->getDoubleValue("ysize", 0.0);