Fix C struct Padding

[lorenzschmid]

Fixes empty name for padding fields in C typdef structure. Error was introduced in initial commit for C typdef structures in #63.

e.g.

typedef struct {
  uint32_t rstn: 1;
  uint32_t : 31;
} plc_ctrl_s;

where the second field has no name and hence, is invalid, becomes

typedef struct {
  uint32_t rstn: 1;
  uint32_t _bits_1_to_31: 31;
} plc_ctrl_s;

Furthermore, this PR signals an error, if C structures are generated and a bit field uses a reserved C keyword.

[lorenzschmid]

Not sure why the jobs are failing. Seems to be an issue with downloading some required tools. The changes proposed here pass the tests locally.