Make a variable size_t to match its use.

lib/zip_source_win32a.c

diff --git a/lib/zip_source_win32a.c b/lib/zip_source_win32a.c
index 85493b6..e343a70 100644
--- a/lib/zip_source_win32a.c
+++ b/lib/zip_source_win32a.c
@@ -90,7 +90,7 @@
 static HANDLE
 _win32_create_temp_a(_zip_source_win32_read_file_t *ctx, void **temp, zip_uint32_t value, PSECURITY_ATTRIBUTES sa)
 {
-    int len;
+    size_t len;
 
     len = strlen((const char *)ctx->fname) + 10;
     if (*temp == NULL) {

lib/zip_source_win32w.c

diff --git a/lib/zip_source_win32w.c b/lib/zip_source_win32w.c
index 551aba5..d8b8f86 100644
--- a/lib/zip_source_win32w.c
+++ b/lib/zip_source_win32w.c
@@ -90,7 +90,7 @@
 static HANDLE
 _win32_create_temp_w(_zip_source_win32_read_file_t *ctx, void **temp, zip_uint32_t value, PSECURITY_ATTRIBUTES sa)
 {
-    int len;
+    size_t len;
 
     len = wcslen((const wchar_t *)ctx->fname) + 10;
     if (*temp == NULL) {