Reduce the initial buffer size for open_memstream (used by vasprintf),
as most strings are usually smaller than that.
Realloc the buffer after finishing the string to further reduce size.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

--- a/libc/stdio/vasprintf.c
+++ b/libc/stdio/vasprintf.c
@@ -39,6 +39,8 @@ int vasprintf(char **__restrict buf, con
 		if (rv < 0) {
 			free(*buf);
 			*buf = NULL;
+		} else {
+			*buf = realloc(*buf, rv + 1);
 		}
 	}
 
--- a/libc/stdio/open_memstream.c
+++ b/libc/stdio/open_memstream.c
@@ -17,6 +17,8 @@
 
 #define COOKIE ((__oms_cookie *) cookie)
 
+#define MEMSTREAM_BUFSIZ	256
+
 typedef struct {
 	char *buf;
 	size_t len;
@@ -134,7 +136,7 @@ FILE *open_memstream(char **__restrict b
 	register FILE *fp;
 
 	if ((cookie = malloc(sizeof(__oms_cookie))) != NULL) {
-		if ((cookie->buf = malloc(cookie->len = BUFSIZ)) == NULL) {
+		if ((cookie->buf = malloc(cookie->len = MEMSTREAM_BUFSIZ)) == NULL) {
 			goto EXIT_cookie;
 		}
 		*cookie->buf = 0;		/* Set nul terminator for buffer. */