build: avoid spurious compiler warning

build: avoid spurious compiler warning

For some weird reason, i686-pc-mingw32-gcc version 4.6.1 at -O2 complained:
../../src/conf/nwfilter_params.c: In function 'virNWFilterVarCombIterCreate':
../../src/conf/nwfilter_params.c:346:23: error: 'minValue' may be used uninitialized in this function [-Werror=uninitialized]
../../src/conf/nwfilter_params.c:319:28: note: 'minValue' was declared here
../../src/conf/nwfilter_params.c:344:23: error: 'maxValue' may be used uninitialized in this function [-Werror=uninitialized]
../../src/conf/nwfilter_params.c:319:18: note: 'maxValue' was declared here
cc1: all warnings being treated as errors

even though all paths of the preceding switch statement either
assign the variables or return.

* src/conf/nwfilter_params.c (virNWFilterVarCombIterAddVariable):
Initialize variables.