dbcaStats() segfaults on disconnected CA links

Reported by Sangwon Yun <email address hidden>:

I faced a segmentation fault from the dbcaStats() function.

The error is happening when link is invalid as below:
record(calc, "$(CBS)SYSM:$(CTRLTYPE)$(IDX)-TEST") {
  field(INPA, "$(CBS)SYSM:$(CTRLTYPE)$(IDX)-TEST1") // non-existing PV
  ...

The error disappeared after adding a check for chid of link as:

    if (pca && ca_state(pca->chid) == cs_conn) {
->
    if (pca && pca->chid && ca_state(pca->chid) == cs_conn) {