Fix Issue #4 - Silence uninitialized warnings

[demerphq]

Since cost is not initialized and the compiler cannot tell
if they are properly initialized (they are) warnings are produced
when building in blead perl under certain compilers.

This patch silences the warnings by making sure the data structures
are initialized to 0.

Warnings silenced:

gcc -c  -I. -D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -Werror=pointer-arith -Werror=vla -Wextra -Wno-long-long -Wno-declaration-after-statement -Wc++-compat -Wwrite-strings -O3   -DVERSION=\"2.101\" -DXS_VERSION=\"2.101\" -fPIC "-I../.."  -Wall -Wno-comment  -DBZ_NO_STDIO  compress.c
compress.c: In function ‘BZ2_compressBlock’:
compress.c:392:54: warning: ‘cost[5]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
	    for (t = 0; t < nGroups; t++) cost[t] += s->len[t][icv];
						  ^~
compress.c:256:11: note: ‘cost[5]’ was declared here
UInt16 cost[BZ_N_GROUPS];
       ^~~~
compress.c:402:21: warning: ‘cost[3]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
	 if (cost[t] < bc) { bc = cost[t]; bt = t; };
	     ~~~~^~~
compress.c:256:11: note: ‘cost[3]’ was declared here
UInt16 cost[BZ_N_GROUPS];
       ^~~~
compress.c:402:21: warning: ‘cost[2]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
	 if (cost[t] < bc) { bc = cost[t]; bt = t; };
	     ~~~~^~~
compress.c:256:11: note: ‘cost[2]’ was declared here
UInt16 cost[BZ_N_GROUPS];
       ^~~~

[demerphq]

I have filed a ticket with the upstream library maintainer

https://sourceware.org/bugzilla/show_bug.cgi?id=28904