pythonmod: add check return value after ftell() (#1478)

Variable 'flen', which might receive a negative value at pythonmod.c:493
by calling function 'ftell', is used without checking at pythonmod.c:508
by calling function 'fread'.

Found by the static analyzer Svace (ISP RAS).

Signed-off-by: Petr Vaganov <petrvaganoff@gmail.com>