editline: Avoid comparison between pointer and zero character constant.

gcc 7.2 warned about this.

ASTERISK-27559

Change-Id: I48960dda9cf0a11b6a9426f775e632363f8caa74

main/editline/term.c

@@ -428,7 +428,7 @@ term_alloc(EditLine *el, const struct termcapstr *t, const char *cap)
          */
 	tlen = 0;
 	for (tmp = tlist; tmp < &tlist[T_str]; tmp++)
-		if (*tmp != NULL && *tmp != '\0' && *tmp != *str) {
+		if (*tmp != NULL && **tmp != '\0' && *tmp != *str) {
 			char *ptr;
 
 			for (ptr = *tmp; *ptr != '\0'; termbuf[tlen++] = *ptr++)