warnings from clang on ARM (FreeBSD on Raspberry Pi)

[Hal Murray]

Eric: Would you please take a look at the second one.  It's a real bug.  You 
removed the 2 lines that setup tvlast on Oct 8.  Were you trying to move them 
out of the "time-critical path"?  (and forgot to put them back in)

I haven't investigated the others.


[  6/121] Compiling libisc/netaddr.c
../../libisc/netaddr.c:145:3: warning: variable 'i' is incremented both in 
the loop header and in the loop body [-Wfor-loop-analysis]
                i++;
                ^
../../libisc/netaddr.c:142:22: note: incremented here
        for (; i < ipbytes; i++) {
                            ^
1 warning generated.

----------

[ 62/121] Compiling libntp/systime.c
../../libntp/systime.c:451:37: warning: variable 'tvlast' is uninitialized 
when used here [-Wuninitialized]
        tvdiff = abs_tval(sub_tval(timetv, tvlast));
                                           ^~~~~~
../../libntp/systime.c:351:2: note: variable 'tvlast' is declared here
        struct timeval timetv, tvlast, tvdiff;
        ^
1 warning generated.

----------

[151/186] Compiling tests/libntp/sfptostr.c
../../tests/libntp/sfptostr.c:31:19: warning: shifting a negative signed 
value is undefined [-Wshift-negative-value]
        s_fp test = -200 << 16; // exact -200.000000
                    ~~~~ ^
../../tests/libntp/sfptostr.c:45:20: warning: shifting a negative signed 
value is undefined [-Wshift-negative-value]
        s_fp test = (-200 << 16) - (1 << 15); // -200 - 0.5
                     ~~~~ ^
../../tests/libntp/sfptostr.c:59:20: warning: shifting a negative signed 
value is undefined [-Wshift-negative-value]
        s_fp test = (-200 << 16) + (1 << 14)*3; // -200 + 0.75
                     ~~~~ ^
3 warnings generated.


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