crush/builder.c: fix realloc handling

crush/builder.c: fix realloc handling

Fix handling of realloc. If realloc() fails it returns NULL, assigning
the return value of realloc() directly to the pointer without checking
for the result will lead to a memory leak in error case.

Use a temporary pointer to hold the result of realloc(). In error case
return -ENOMEM, otherwise assign it to the pointer we want to realloc.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>