When xmallocoverrun check is enabled, everything allocated with xmalloc() should be
passed to xfree(), not free(). Otherwise glibc will detect an inconsistency in the
internal malloc data structures and abort the program.

Due to this, Emacs configured with --enable-checking=xmallocoverrun currently crashes,
both with default (gtk) and lucid toolkits. Here is an obvious fix for all of these issues.

Dmitry