tree-wide: check memstream buffer after closing the handle

When closing the FILE handle attached to a memstream, it may attempt to
do a realloc() that may fail during OOM situations, in which case we are
left with the buffer pointer pointing to NULL and buffer size > 0. For
example:

```
    #include <errno.h>
    #include <stdio.h>
    #include <stdlib.h>

    void *realloc(void *ptr, size_t size) {
        return NULL;
    }

    int main(int argc, char *argv[])
    {
        FILE *f;
        char *buf;
        size_t sz = 0;

        f = open_memstream(&buf, &sz);
        if (!f)
            return -ENOMEM;

        fputs("Hello", f);

        fflush(f);
        printf("buf: 0x%lx, sz: %lu, errno: %d\n",
                    (unsigned long) buf, sz, errno);
        fclose(f);
        printf("buf: 0x%lx, sz: %lu, errno: %d\n",
                    (unsigned long) buf, sz, errno);

        return 0;
    }
```

```
$ gcc -o main main.c
$ ./main
buf: 0x74d4a0, sz: 5, errno: 0
buf: 0x0, sz: 5, errno: 0
```

This might do unexpected things if the underlying code expects a valid
pointer to the memstream buffer after closing the handle.

Found by Nallocfuzz.

src/basic/string-util.c

@@ -9,6 +9,7 @@
 #include "alloc-util.h"
 #include "escape.h"
 #include "extract-word.h"
+#include "fd-util.h"
 #include "fileio.h"
 #include "gunicode.h"
 #include "locale-util.h"
@@ -603,9 +604,9 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
                 STATE_CSI,
                 STATE_CSO,
         } state = STATE_OTHER;
-        char *obuf = NULL;
+        _cleanup_free_ char *obuf = NULL;
+        _cleanup_fclose_ FILE *f = NULL;
         size_t osz = 0, isz, shift[2] = {}, n_carriage_returns = 0;
-        FILE *f;
 
         assert(ibuf);
         assert(*ibuf);
@@ -713,11 +714,13 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
                 }
         }
 
-        if (fflush_and_check(f) < 0) {
-                fclose(f);
-                return mfree(obuf);
-        }
-        fclose(f);
+        if (fflush_and_check(f) < 0)
+                return NULL;
+
+        f = safe_fclose(f);
+
+        if (!obuf)
+                return NULL;
 
         free_and_replace(*ibuf, obuf);