Check zalloc return for out of memory situation

Most zalloc calls in weston are checked, this fixes a handful that were
being ignored.  As found by `grep -EIsr "[^x]zalloc\(" . -A1`

Signed-off-by: Bryce Harrington <b.harrington@samsung.com>
diff --git a/src/compositor-wayland.c b/src/compositor-wayland.c
index f35db9c..67f15be 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -256,6 +256,12 @@
 	}
 
 	sb = zalloc(sizeof *sb);
+	if (sb == NULL) {
+		weston_log("could not zalloc %ld memory for sb: %m\n", sizeof *sb);
+		close(fd);
+		free(data);
+		return NULL;
+	}
 
 	sb->output = output;
 	wl_list_init(&sb->free_link);