Prevent undefined behavior with --cpp-lines (#29041)

Fixes an issue where `--cpp-lines` could attempt to index an empty
string with `-1`.

In some cases, `cp` could be `""` and `strlen(cp)` would be 0. This
means we would attempt to index the array with a negative value
(undefined behavior!)

[Reviewed by @arifthpe]

Author: jabraham17

compiler/backend/beautify.cpp

@@ -227,11 +227,8 @@ void beautify(fileinfo* origfile) {
         }
         fprintf(outputfile, ZLINEFORMAT, zlineP, zname);
       }
-    }
-
-    if (cp[strlen(cp)-1] == '\n')
-      new_line = TRUE;
-    else {
+      new_line = cp[strlen(cp)-1] == '\n';
+    } else {
       new_line = FALSE;
     }