stdio: simplify vasprintf()

... by removing unneeded NULL check, as free() already does it.
By the way, we don't need to set a stack variable back to NULL.

Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>

Change-Id: Id90eb8f042b5c922c5ff139b11ff8366fb404566
diff --git a/libc/stdio/vasprintf.c b/libc/stdio/vasprintf.c
index c3280c9..54c46b3 100644
--- a/libc/stdio/vasprintf.c
+++ b/libc/stdio/vasprintf.c
@@ -48,10 +48,7 @@
 	return (ret);
 
 err:
-	if (f._bf._base) {
-		free(f._bf._base);
-		f._bf._base = NULL;
-	}
+	free(f._bf._base);
 	*str = NULL;
 	errno = ENOMEM;
 	return (-1);