s_fp_add: completely init c after add

This was reported in clamav via a "coverity report" #11125/#11126. I can't
get the coverity report and I've been told that

|< linnatic> bigeasy: unfortunately, I can't get you the coverity report
|< linnatic> bigeasy: but it does seem that the patch is not
|required as the possible coverity issue is unlikely to occur

so if you thing that this patch is not required (which is what I assume) then
please drop that patch, I just added since it was in the clamav tree…

[sebastian@breakpoint: patch description]
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

src/addsub/s_fp_add.c

@@ -16,7 +16,7 @@ void s_fp_add(fp_int *a, fp_int *b, fp_int *c)
   register fp_word  t;
 
   y       = MAX(a->used, b->used);
-  oldused = c->used;
+  oldused = MIN(c->used, FP_SIZE);
   c->used = y;
  
   t = 0;