BUG/MEDIUM: lua: fix memory leaks with realloc() on non-glibc systems

BUG/MEDIUM: lua: fix memory leaks with realloc() on non-glibc systems

In issue #1406, Lev Petrushchak reported a nasty memory leak on Alpine
since haproxy 2.4 when using Lua, that memory profiling didn't detect.
After inspecting the code and Lua's code, it appeared that Lua's default
allocator does an explicit free() on size zero, while since 2.4 commit
d36c7fa5e ("MINOR: lua: simplify hlua_alloc() to only rely on realloc()"),
haproxy only calls realloc(ptr,0) that performs a free() on glibc but not
on other systems as it's not required by POSIX...

This patch reinstalls the explicit test for nsize==0 to call free().

Thanks to Lev for the very documented report, and to Tim for the links
to a musl thread on the same subject that confirms the diagnostic.

This must be backported to 2.4.