Fix gcc -Wconversion warnings

Those are gone now:
asmsx.c: In function 'register_label':
asmsx.c:465:26: warning: conversion to 'int' from 'size_t' may alter its
value [-Wconversion]
asmsx.c:466:2: warning: conversion to 'size_t' from 'int' may change the
sign of the result [-Wsign-conversion]
asmsx.c: In function 'register_local':
asmsx.c:489:26: warning: conversion to 'int' from 'size_t' may alter its
value [-Wconversion]
asmsx.c:490:2: warning: conversion to 'size_t' from 'int' may change the
sign of the result [-Wsign-conversion]
asmsx.c: In function 'register_symbol':
asmsx.c:519:26: warning: conversion to 'int' from 'size_t' may alter its
value [-Wconversion]
asmsx.c:520:2: warning: conversion to 'size_t' from 'int' may change the
sign of the result [-Wsign-conversion]
asmsx.c: In function 'register_variable':
asmsx.c:547:26: warning: conversion to 'int' from 'size_t' may alter its
value [-Wconversion]
asmsx.c:548:2: warning: conversion to 'size_t' from 'int' may change the
sign of the result [-Wsign-conversion]
asmsx.c: In function 'write_zx_byte':
asmsx.c:725:9: warning: conversion to 'char' from 'int' may alter its
value [-Wconversion]
asmsx.c: In function 'cas_write_file':
asmsx.c:1202:19: warning: conversion to 'int' from 'size_t' may alter
its value [-Wconversion]
asmsx.c: In function 'main':
asmsx.c:1265:28: warning: conversion to 'int' from 'size_t' may alter
its value [-Wconversion]