linux-user: Fix possible realloc memory leak

Extract from "man realloc":
"If realloc() fails the original block is left untouched;
it is not freed or moved."

Fix a possible memory leak (reported by cppcheck).

Cc: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Riku Voipio <riku.voipio@nokia.com>