fmemopen: add a NUL byte when writing beyond current buffer end

This behaviour is not currently mandated by the POSIX spec, but is what
glibc does and will be part of the next POSIX standard according to
https://austingroupbugs.net/view.php?id=657#c6535

Author: arichardson

newlib/libc/tinystdio/fmemopen.c

@@ -62,6 +62,13 @@ __fmem_put(char c, FILE *f)
         mf->buf[mf->pos++] = c;
         if (mf->pos > mf->size) {
             mf->size = mf->pos;
+            /* When a stream open for update (the mode argument includes '+') or
+             * for writing only is successfully written and the write advances
+             * the current buffer end position, a null byte shall be written at
+             * the new buffer end position if it fits. */
+            if (mf->size < mf->bufsize) {
+                mf->buf[mf->size] = '\0';
+            }
         }
         return (unsigned char)c;
     } else {