[midx] Fix an undefined behavior (left-shift signed overflow)

There was a missing check to ensure that the `off64_t` (which is a
signed value) didn't overflow when parsing it from the midx file. This
shouldn't have huge repercusions since the parsed value is immediately
validated afterwards, but then again, there is no such thing as "benign"
undefined behavior.

This change makes all the bitwise arithmetic happen with unsigned types
and is only casted to `off64_t` until the very end.

Thanks to Taotao Gu for finding and reporting this!

src/libgit2/midx.c

@@ -225,8 +225,13 @@ int git_midx_parse(
 	chunk_hdr = data + sizeof(struct git_midx_header);
 	last_chunk = NULL;
 	for (i = 0; i < hdr->chunks; ++i, chunk_hdr += 12) {
-		chunk_offset = ((off64_t)ntohl(*((uint32_t *)(chunk_hdr + 4)))) << 32 |
-				((off64_t)ntohl(*((uint32_t *)(chunk_hdr + 8))));
+		uint32_t chunk_id = ntohl(*((uint32_t *)(chunk_hdr + 0)));
+		uint64_t high_offset = ((uint64_t)ntohl(*((uint32_t *)(chunk_hdr + 4)))) & 0xffffffffu;
+		uint64_t low_offset = ((uint64_t)ntohl(*((uint32_t *)(chunk_hdr + 8)))) & 0xffffffffu;
+
+		if (high_offset >= INT32_MAX)
+			return midx_error("chunk offset out of range");
+		chunk_offset = (off64_t)(high_offset << 32 | low_offset);
 		if (chunk_offset < last_chunk_offset)
 			return midx_error("chunks are non-monotonic");
 		if (chunk_offset >= trailer_offset)
@@ -235,7 +240,7 @@ int git_midx_parse(
 			last_chunk->length = (size_t)(chunk_offset - last_chunk_offset);
 		last_chunk_offset = chunk_offset;
 
-		switch (ntohl(*((uint32_t *)(chunk_hdr + 0)))) {
+		switch (chunk_id) {
 		case MIDX_PACKFILE_NAMES_ID:
 			chunk_packfile_names.offset = last_chunk_offset;
 			last_chunk = &chunk_packfile_names;