Fix compiler warnings on Fedora 26 / GCC 7.

GCC 7 has added capability to produce warnings, this fixes most of those
warnings.  The specific warnings are disabled in a few places:

* app_voicemail.c: truncation of paths more than 4096 chars in many places.
* chan_mgcp.c: callid truncated to 80 chars.
* cdr.c: two userfields are combined to cdr copy, fix would break ABI.
* tcptls.c: ignore use of deprecated method SSLv3_client_method().

ASTERISK-27156 #close

Change-Id: I65f280e7d3cfad279d16f41823a4d6fddcbc4c88

main/utils.c

@@ -1530,7 +1530,7 @@ char *ast_strsep(char **iss, const char sep, uint32_t flags)
 	int found = 0;
 	char stack[8];
 
-	if (iss == NULL || *iss == '\0') {
+	if (ast_strlen_zero(st)) {
 		return NULL;
 	}