s3/lib: clang: Fix 'Value stored to 'b' is never read'

s3/lib: clang: Fix 'Value stored to 'b' is never read'

Fixes:

source3/lib/substitute.c:516:7: warning: Value stored to 'b' is never read <--[clang]
        for (b = s = a_string; (p = strchr_m(s, '%')); s = a_string + (p - b)) {
             ^   ~~~~~~~~~~~~
source3/lib/substitute.c:709:7: warning: Value stored to 'b' is never read <--[clang]
        for (b = s = a_string; (p = strchr_m(s, '%')); s = a_string + (p - b)) {
             ^   ~~~~~~~~~~~~
source3/lib/substitute.c:811:7: warning: Value stored to 'b' is never read <--[clang]
        for (b = s = a_string; (p = strchr_m(s, '%')); s = a_string + (p - b)) {
             ^   ~~~~~~~~~~~~
3 warnings generated.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>