bsd-user: Fix possible memory leaks and wrong realloc call

These errors were reported by cppcheck:

[bsd-user/elfload.c:1108]: (error) Common realloc mistake: "syms" nulled but not freed upon failure
[bsd-user/elfload.c:1076]: (error) Memory leak: s
[bsd-user/elfload.c:1079]: (error) Memory leak: syms

v2:
* The previous fix for memory leaks was incomplete (thanks to Peter Maydell for te hint).
* Fix wrong realloc usage, too.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
