Use 64bit arithmetic to avoid overflow

The multiplication is evaluated using 32bit arithmetic, and then stored
in a 64bit integer. The 32bit integer overflow can be avoided by casting
to a 64bit type first.

src/serial_posix.c

@@ -691,7 +691,7 @@ dc_serial_read (dc_iostream_t *abstract, void *data, size_t size, size_t *actual
 
 			if (init) {
 				// Calculate the initial timeout.
-				timeout = device->timeout * 1000;
+				timeout = (dc_usecs_t) device->timeout * 1000;
 				// Calculate the target time.
 				target = now + timeout;
 				init = 0;