Fix -Werror=conversion in available_valid_utf_8
diff --git a/internal/cgen/builtin.go b/internal/cgen/builtin.go
index e4eff2a..9231b28 100644
--- a/internal/cgen/builtin.go
+++ b/internal/cgen/builtin.go
@@ -174,7 +174,7 @@
 			return err
 		}
 		b.printf("((uint64_t)(wuffs_base__utf_8__longest_valid_prefix(%s%s,\n"+
-			"((size_t)(wuffs_base__u64__min(%s%s - %s%s, ",
+			"((size_t)(wuffs_base__u64__min(((uint64_t)(%s%s - %s%s)), ",
 			iopPrefix, name, io2Prefix, name, iopPrefix, name)
 		if err := g.writeExpr(b, args[0].AsArg().Value(), depth); err != nil {
 			return err
diff --git a/release/c/wuffs-unsupported-snapshot.c b/release/c/wuffs-unsupported-snapshot.c
index 9b9c9e1..dcb49d4 100644
--- a/release/c/wuffs-unsupported-snapshot.c
+++ b/release/c/wuffs-unsupported-snapshot.c
@@ -16700,7 +16700,7 @@
               }
               v_n64 = wuffs_base__u64__min(v_string_length, 65535);
               v_n64 = ((uint64_t)(wuffs_base__utf_8__longest_valid_prefix(iop_a_src,
-                  ((size_t)(wuffs_base__u64__min(io2_a_src - iop_a_src, v_n64))))));
+                  ((size_t)(wuffs_base__u64__min(((uint64_t)(io2_a_src - iop_a_src)), v_n64))))));
               v_token_length = ((uint32_t)((v_n64 & 65535)));
               if (v_token_length <= 0) {
                 if ((a_src && a_src->meta.closed) || (((uint64_t)(io2_a_src - iop_a_src)) >= 4)) {