Fix off-by-1 bug in --enable-checking=stringbytes

Fix off-by-1 bug in --enable-checking=stringbytes

Evidently nobody builds Emacs with --enable-checking=all,
which is no surprise as it is so slow as to be unusable nowadays.
Perhaps we should remove the slowest checks, or move them into
another category, or speed them up, or something.
* src/alloc.c (SDATA_SIZE) [GC_CHECK_STRING_BYTES]: Fix off-by-one
error in size calculation, which caused a failure when
--enable-checking=stringbytes was used.  I introduced this bug in
2016-09-08T01:08:45!eggert@cs.ucla.edu "Port flexible array
members to GCC + valgrind".