common/fd.cc: fix possible out-of-bounds write

Read max 'sizeof(target) - 1' to not write out of bound
later on the 'target[r] = 0;' call in case we read the
full PATH_MAX.

CID 1128416 (#1 of 1): Out-of-bounds write (OVERRUN)
 overrun-local: Overrunning array target of 4096 bytes
 at byte offset 4096 using index r (which evaluates to 4096).

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>