store: don't underflow pointer

when number of snips is 0, `end - 1` will go one below the
buffer. computing out of bound pointer (with the exception of
one past the end) is technically UB [0] so avoid it by doing the
computation on index instead of pointer.

0: https://port70.net/~nsz/c/c11/n1570.html#6.5.6p9

src/store.c

@@ -3,6 +3,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
+#include <stddef.h>
 #include <limits.h>
 #include <stdbool.h>
 #include <stdio.h>
@@ -567,18 +568,19 @@ bool cs_snip_iter(struct ref_guard *guard, enum cs_iter_direction direction,
     }
 
     struct cs_snip *start = guard->cs->snips;
-
-    // cppcheck-suppress [constVariablePointer,unmatchedSuppression]
-    // TODO: False positive? Report upstream
-    struct cs_snip *end = start + guard->cs->header->nr_snips;
-
+    ptrdiff_t idx, nr_snips = (ptrdiff_t)guard->cs->header->nr_snips;
     if (*snip) {
-        *snip = *snip + (direction == CS_ITER_NEWEST_FIRST ? -1 : 1);
+        ptrdiff_t cur_idx = *snip - start;
+        idx = cur_idx + (direction == CS_ITER_NEWEST_FIRST ? -1 : 1);
     } else {
-        *snip = direction == CS_ITER_NEWEST_FIRST ? end - 1 : start;
+        idx = direction == CS_ITER_NEWEST_FIRST ? nr_snips - 1 : 0;
     }
 
-    return *snip >= start && *snip < end;
+    if (idx >= 0 && idx < nr_snips) {
+        *snip = start + idx;
+        return true;
+    }
+    return false;
 }
 
 /**