osd: humor coverity

osd: humor coverity

CID 1058392: Out-of-bounds access (OVERRUN_DYNAMIC) [select issue]

CID 1058391 (#1 of 1): Out-of-bounds access (OVERRUN)
32. alloc_strlen: Allocating insufficient memory for the terminating null of the string.

CID 1058390 (#1 of 1): Unchecked return value from library (CHECKED_RETURN)
13. check_return: Calling function "this->class_handler->open_all_classes()" without checking return value. It wraps a library function that may fail and return an error code. [show details]
    	14. unchecked_value: No check of the return value of "this->class_handler->open_all_classes()".

Signed-off-by: Sage Weil <sage@inktank.com>