[PATCH v2] src/swapon.c: initiate p to NULL

[Sun Ke <sunke32@xxxxxxxxxx>]

when run make, show:
swapon.c:135:3: warning: 'p' may be used uninitialized in this function
[-Wmaybe-uninitialized]
   memcpy(p, buf, BUF_SIZE);
      ^~~~~~~~~~~~~~~~~~~~~~~~

This's a fake uninitialized warning.
gcc think the 'p' can be used uninitialized at here when verb is
TEST_MWRITE_BEFORE_AND_MWRITE_AFTER:
    case TEST_MWRITE_BEFORE_AND_MWRITE_AFTER:
	memcpy(p, buf, BUF_SIZE);
	break;

But, if verb is TEST_MWRITE_BEFORE_AND_MWRITE_AFTER, the 'p' will be
initialized by:
    switch (verb) {
    case TEST_MWRITE_BEFORE_AND_MWRITE_AFTER:
    case TEST_MWRITE_BEFORE:
	p = mmap(NULL, BUF_SIZE, PROT_WRITE | PROT_READ, MAP_SHARED,

Shall we make the build warning disappear?

Signed-off-by: Sun Ke <sunke32@xxxxxxxxxx>
---
V2: Improve the commit message.
---
 src/swapon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/swapon.c b/src/swapon.c
index afaed405..fe8949a3 100644
--- a/src/swapon.c
+++ b/src/swapon.c
@@ -41,7 +41,7 @@ int main(int argc, char **argv)
 		.sa_handler	= handle_signal,
 	};
 	enum verbs verb = TEST_SWAPON;
-	void *p;
+	void *p = NULL;
 	ssize_t sz;
 	int fd = -1;
 	int ret, c;
-- 
2.13.6