[libc] Fix casts for arm32 after Wconversion (#129771)

Followup to #127523

There were some test failures on arm32 after enabling Wconversion. There
were some tests that were failing due to missing casts. Also I changed
BigInt's `safe_get_at` back to being signed since it needed the ability
to be negative.

1 files changed, 2 insertions, 1 deletions

diff --git a/libc/src/string/memory_utils/generic/byte_per_byte.h b/libc/src/string/memory_utils/generic/byte_per_byte.h
index 2aecf0126cc4..862aeecab7f5 100644
--- a/libc/src/string/memory_utils/generic/byte_per_byte.h
+++ b/libc/src/string/memory_utils/generic/byte_per_byte.h
@@ -38,7 +38,8 @@ inline_memmove_byte_per_byte(Ptr dst, CPtr src, size_t count) {
       dst[offset] = src[offset];
   } else {
     LIBC_LOOP_NOUNROLL
-    for (ptrdiff_t offset = count - 1; offset >= 0; --offset)
+    for (ptrdiff_t offset = static_cast<ptrdiff_t>(count - 1); offset >= 0;
+         --offset)
       dst[offset] = src[offset];
   }
 }