perf top: Remove needless malloc(0) call that triggers -Walloc-size

GCC 14 introduces a new -Walloc-size included in -Wextra which errors out
like:

builtin-top.c: In function ‘prompt_integer’:
builtin-top.c:360:21: error: allocation of insufficient size ‘0’ for
type ‘char’ with size ‘1’ [-Werror=alloc-size]
360 | char *buf = malloc(0), *p;
| ^~~~~~

Just set it to NULL, getline() will do the allocation.

Signed-off-by: Sun Haiyong <[e-mail]>
Signed-off-by: Yanteng Si <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[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/builtin-top.c

···
 
 static void prompt_integer(int *target, const char *msg)
 {
-	char *buf = malloc(0), *p;
+	char *buf = NULL, *p;
 	size_t dummy = 0;
 	int tmp;