FS-7866: [core] fix crash when running incorrect var api expansion syntax "eval…

FS-7866: [core] fix crash when running incorrect var api expansion syntax "eval ${${external_sip_ip}:4}"

src/switch_event.c

@@ -2365,6 +2365,7 @@ SWITCH_DECLARE(char *) switch_event_expand_headers_check(switch_event_t *event,
 						}
 					}
 
+					if (sub_val) {
 						if (offset || ooffset) {
 							cloned_sub_val = strdup(sub_val);
 							switch_assert(cloned_sub_val);
@@ -2382,7 +2383,7 @@ SWITCH_DECLARE(char *) switch_event_expand_headers_check(switch_event_t *event,
 								*ptr = '\0';
 							}
 						}
-
+					}
 
 					switch_safe_free(expanded);
 				} else {