Fix -Wsign-conversion in bigint::subtract_aligned.

n is assigned a size_t, and only used for comparisons with j.

j is assigned 0, compared to n (size_t), and passed to basic_memory_buffer::operator[] (size_t).

include/fmt/format-inl.h

@@ -526,7 +526,7 @@ class bigint {
     FMT_ASSERT(compare(*this, other) >= 0, "");
     bigit borrow = 0;
     int i = other.exp_ - exp_;
-    for (int j = 0, n = static_cast<int>(other.bigits_.size()); j != n;
+    for (size_t j = 0, n = other.bigits_.size(); j != n;
          ++i, ++j) {
       subtract_bigits(i, other.bigits_[j], borrow);
     }