build: fix uninitialized variables

Build for Fedora 34 is breaking out due to uninitialized variables in
 a few places:
 For example,
 [100%] Built target merger.test
/source/build/usr/src/debug/tarantool-2.9.0.116/src/box/sql.c: In function 'tarantoolSqlNextSeqId':
/source/build/usr/src/debug/tarantool-2.9.0.116/src/box/sql.c:1186:13: error: 'key' may be used uninitialized [-Werror=maybe-uninitialized]
 1186 |         if (box_index_max(BOX_SEQUENCE_ID, 0 /* PK */, key,

Needed for: #6074

src/box/sql.c

@@ -1180,9 +1180,10 @@ sql_debug_info(struct info_handler *h)
 int
 tarantoolSqlNextSeqId(uint64_t *max_id)
 {
-	char key[16];
+	char key[1];
 	struct tuple *tuple;
 	char *key_end = mp_encode_array(key, 0);
+	assert(key_end - key == 1);
 	if (box_index_max(BOX_SEQUENCE_ID, 0 /* PK */, key,
 			  key_end, &tuple) != 0)
 		return -1;

test/box/function1.c

@@ -259,6 +259,7 @@ test_return_mp(box_function_ctx_t *ctx, const char *args, const char *args_end)
 	(void) args;
 	(void) args_end;
 	char buf[512];
+	memset(buf, '\0', 512);
 	char *pos = mp_encode_uint(buf, 1);
 	int rc = box_return_mp(ctx, buf, pos);
 	if (rc != 0)

test/sql-tap/gh-6024-funcs-return-bin.c

@@ -14,6 +14,7 @@ ret_bin(box_function_ctx_t *ctx, const char *args, const char *args_end)
 	(void)args_end;
 	const char bin[] = "some varbinary string";
 	char res[BUF_SIZE];
+	memset(res, '\0', BUF_SIZE);
 	char *end = mp_encode_bin(res, bin, sizeof(bin));
 	box_return_mp(ctx, res, end);
 	return 0;
@@ -27,6 +28,7 @@ ret_uuid(box_function_ctx_t *ctx, const char *args, const char *args_end)
 	struct tt_uuid uuid;
 	memset(&uuid, 0x11, sizeof(uuid));
 	char res[BUF_SIZE];
+	memset(res, '\0', BUF_SIZE);
 	char *end = mp_encode_uuid(res, &uuid);
 	box_return_mp(ctx, res, end);
 	return 0;
@@ -40,6 +42,7 @@ ret_decimal(box_function_ctx_t *ctx, const char *args, const char *args_end)
 	decimal_t dec;
 	decimal_from_string(&dec, "9999999999999999999.9999999999999999999");
 	char res[BUF_SIZE];
+	memset(res, '\0', BUF_SIZE);
 	char *end = mp_encode_decimal(res, &dec);
 	box_return_mp(ctx, res, end);
 	return 0;

test/sql-tap/sql_uuid.c

@@ -27,6 +27,7 @@ is_uuid(box_function_ctx_t *ctx, const char *args, const char *args_end)
 	}
 
 	char res[BUF_SIZE];
+	memset(res, '\0', BUF_SIZE);
 	char *end = mp_encode_bool(res, is_uuid);
 	box_return_mp(ctx, res, end);
 	return 0;
@@ -40,6 +41,7 @@ ret_uuid(box_function_ctx_t *ctx, const char *args, const char *args_end)
 	struct tt_uuid uuid;
 	memset(&uuid, 0x11, sizeof(uuid));
 	char res[BUF_SIZE];
+	memset(res, '\0', BUF_SIZE);
 	char *end = mp_encode_uuid(res, &uuid);
 	box_return_mp(ctx, res, end);
 	return 0;