Fix T47252: FileBrowser: buffer overflow with scripts defining too long 'filter_glob' string.

Fixed this with three changes:
* filter_glob is now 255 char max (63 could be a bit limited in some rare cases).
* IO templates now explicitely define max len of that property (such that scripters are aware of the limit).
* ED_fileselect_set_params() is now safe regarding too long strings from a 'filter_glob' op property.

release/scripts/templates_py/operator_file_export.py

@@ -28,6 +28,7 @@ class ExportSomeData(Operator, ExportHelper):
     filter_glob = StringProperty(
             default="*.txt",
             options={'HIDDEN'},
+            maxlen=255,  # Max internal buffer length, longer would be clamped.
             )
 
     # List of operator properties, the attributes will be assigned

release/scripts/templates_py/operator_file_import.py

@@ -31,6 +31,7 @@ class ImportSomeData(Operator, ImportHelper):
     filter_glob = StringProperty(
             default="*.txt",
             options={'HIDDEN'},
+            maxlen=255,  # Max internal buffer length, longer would be clamped.
             )
 
     # List of operator properties, the attributes will be assigned