restore(8): Prevent some heap overflows

restore(8): Prevent some heap overflows

The environment variable TMPDIR was copied unchecked into a fixed-size heap
buffer.  Use a length-limiting snprintf in place of ordinary sprintf to
prevent the overflow.  Long TMPDIR variables can still cause odd truncated
filenames, which may be undesirable.

Reported by:	Coverity (CWE-120)
CIDs:		1006706, 1006707
Sponsored by:	Dell EMC Isilon