zlib: fix compilation failures with Sun C Compilaer

zlib: fix compilation failures with Sun C Compilaer

Do this by removing a couple of useless return statements.  Without this
change, compilation with Sun C Compiler 5.9 (SunOS_i386 Patch 124868-15
2010/08/11) fails with the following message:

  "zlib.c", line 192: void function cannot return value
  "zlib.c", line 201: void function cannot return value
  cc: acomp failed for zlib.c

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>