Fix a memory leak on mesh_attr_text() error path

Fix a memory leak on mesh_attr_text() error path

Should there not be enough room in the output buffer, the
bss_basic_rate_set line would not be printed. This error case was
handled otherwise, but the temporary memory allocation for building the
information was not freed.

Signed-off-by: Jouni Malinen <j@w1.fi>