avoid look past std::vector, as identified by cran valgrind (#5426)

* avoid look past std::vector, as identified by cran valgrind
Co-authored-by: Lionel Henry <[email protected]>

Author: romainfrancois

src/group_by.cpp

@@ -178,7 +178,9 @@ class VectorExpander : public Expander {
       for (R_xlen_t j = start; j < end;) {
         R_xlen_t current = vec_pos[j];
         R_xlen_t start_idx = j;
-        while (j < end && vec_pos[++j] == current);
+
+        ++j;
+        for (; j < end && vec_pos[j] == current; ++j);
         expanders.push_back(expander(data_, positions_, depth_ + 1, current, start_idx, j));
       }
     }