This also has some minor cleanups.
* lib/canonicalize-lgpl.c, lib/canonicalize.c: No need to include
stddef.h, since the code no longer refers directly to ptrdiff_t.
* lib/canonicalize-lgpl.c (realpath_stk):
* lib/canonicalize.c (canonicalize_filename_mode_stk):
Treat size overflow like other out-of-memory.
* lib/canonicalize.c: No need to include stdlib.h, since
the code no longer refers to stdlib.h functions (other
than those that canonicalize.h must declare).
* lib/canonicalize.c (canonicalize_filename_mode_stk):
Do not bother terminating the string result on error.