selftests/bpf: fix few more compiler warnings

When compiling with -O2, GCC detects few problems with selftests/bpf, so
fix all of them. Two are real issues (uninitialized err and nums
out-of-bounds access), but two other uninitialized variables warnings
are due to GCC not being able to prove that variables are indeed
initialized under conditions under which they are used.

Fix all 4 cases, though.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c

@@ -329,7 +329,7 @@ static int get_syms(char ***symsp, size_t *cntp)
 	struct hashmap *map;
 	char buf[256];
 	FILE *f;
-	int err;
+	int err = 0;
 
 	/*
 	 * The available_filter_functions contains many duplicates,
@@ -404,7 +404,7 @@ static void test_bench_attach(void)
 	double attach_delta, detach_delta;
 	struct bpf_link *link = NULL;
 	char **syms = NULL;
-	size_t cnt, i;
+	size_t cnt = 0, i;
 
 	if (!ASSERT_OK(get_syms(&syms, &cnt), "get_syms"))
 		return;

tools/testing/selftests/bpf/prog_tests/usdt.c

@@ -12,7 +12,7 @@ int lets_test_this(int);
 
 static volatile int idx = 2;
 static volatile __u64 bla = 0xFEDCBA9876543210ULL;
-static volatile short nums[] = {-1, -2, -3, };
+static volatile short nums[] = {-1, -2, -3, -4};
 
 static volatile struct {
 	int x;

tools/testing/selftests/bpf/prog_tests/xdp_synproxy.c

@@ -63,7 +63,7 @@ static bool expect_str(char *buf, size_t size, const char *str, const char *name
 static void test_synproxy(bool xdp)
 {
 	int server_fd = -1, client_fd = -1, accept_fd = -1;
-	char *prog_id, *prog_id_end;
+	char *prog_id = NULL, *prog_id_end;
 	struct nstoken *ns = NULL;
 	FILE *ctrl_file = NULL;
 	char buf[CMD_OUT_BUF_SIZE];