Fix for NULL pointer dereference for empty writables
A writable whose default text is deleted but whose length was set would fault when the Resource file was subsequently reloaded. window/gui.c: Adjust logic to only call strlen() on non-NULL strings. misc/gui.c, menu/gui.c: Copy similar fix to these files. Fixes issue described https://www.riscosopen.org/forum/forums/4/topics/15011