libsframe: avoid using magic number

libsframe: avoid using magic number

Define a new constant for the maximum number of stack offsets handled in
libsframe, and use it.  Note that the SFrame format does not define such
a constant (limit).  This is an implmentation-defined constant in
libsframe.

include/
	* sframe-api.h (MAX_NUM_STACK_OFFSETS): New definition.
libsframe/
	* sframe.c (sframe_fre_sanity_check_p): Use it.