perf config: Fix caching and memory leak in perf_home_perfconfig()

Acaict, perf_home_perfconfig() is supposed to cache the result of
home_perfconfig, which returns the default location of perfconfig for
the user, given the HOME environment variable.

However, the current implementation calls home_perfconfig every time
perf_home_perfconfig() is called (so no caching is actually performed),
replacing the previous pointer, thus also causing a memory leak.

This patch adds a check of whether either config or failed is set and,
in that case, directly returns config without calling home_perfconfig at
each invocation.

Fixes: f5f03e19ce14fc31 ("perf config: Add perf_home_perfconfig function")
Signed-off-by: Riccardo Mancini <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jin Yao <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Song Liu <[e-mail]>
Link: http://lore.kernel.org/lkml/[e-mail]
[ Removed needless double check for the 'failed' variable ]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/config.c

···
 	static const char *config;
 	static bool failed;
 
-	config = failed ? NULL : home_perfconfig();
+	if (failed || config)
+		return config;
+
+	config = home_perfconfig();
 	if (!config)
 		failed = true;