General: Avoid implicit conversion to char when changes value to negative.

clang 5.0 warned about this.

ASTERISK-27557

Change-Id: I7cceaa88e147cbdf81a3a7beec5c1c20210fa41e

apps/app_sms.c

@@ -807,7 +807,7 @@ static void sms_log(sms_t * h, char status)
 					*p++ = '\\';
 					*p++ = 'r';
 				} else if (h->ud[n] < 32 || h->ud[n] == 127) {
-					*p++ = 191;
+					*p++ = 0xbf;
 				} else {
 					*p++ = h->ud[n];
 				}