perf string: Avoid undefined NULL+1

While the value NULL+1 is never used it triggers a ubsan warning.
Restructure and comment the loop to avoid this.

Reviewed-by: James Clark <[e-mail]>
Signed-off-by: Ian Rogers <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Kan Liang <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/string.c

···
 
 	do {
 		ptr = strpbrk(str, stopset);
-		if (ptr == str ||
-		    (ptr == str + 1 && *(ptr - 1) != '\\'))
+		if (!ptr) {
+			/* stopset not in str. */
 			break;
+		}
+		if (ptr == str) {
+			/* stopset character is first in str. */
+			break;
+		}
+		if (ptr == str + 1 && str[0] != '\\') {
+			/* stopset chacter is second and wasn't preceded by a '\'. */
+			break;
+		}
 		str = ptr + 1;
-	} while (ptr && *(ptr - 1) == '\\' && *(ptr - 2) != '\\');
+	} while (ptr[-1] == '\\' && ptr[-2] != '\\');
 
 	return ptr;
 }