Fix signedness warnings (-Wsign-conversion, gcc)

lib/zip_source_read.c

diff --git a/lib/zip_source_read.c b/lib/zip_source_read.c
index 88edd65..28c0834 100644
--- a/lib/zip_source_read.c
+++ b/lib/zip_source_read.c
@@ -64,7 +64,7 @@
 		return -1;
 	    }
 	    else {
-		return bytes_read;
+		return (zip_int64_t)bytes_read;
 	    }
 	}