HACKING: document preference for g_new instead of g_malloc

This patch documents the preference for g_new instead of g_malloc. The
reasons were adapted from commit b45c03f585ea9bb1af76c73e82195418c294919d.

Discussion in QEMU's mailing list:
http://lists.nongnu.org/archive/html/qemu-devel/2018-05/msg03238.html

Backports commit f7c922ed3d8e3cb54febbdc594ce9f4400e0d290 from qemu

qemu/HACKING

@@ -100,6 +100,15 @@ Please note that g_malloc will exit on allocation failure, so there
 is no need to test for failure (as you would have to with malloc).
 Calling g_malloc with a zero size is valid and will return NULL.
 
+Prefer g_new(T, n) instead of g_malloc(sizeof(T) * n) for the following
+reasons:
+
+  a. It catches multiplication overflowing size_t;
+  b. It returns T * instead of void *, letting compiler catch more type
+     errors.
+
+Declarations like T *v = g_malloc(sizeof(*v)) are acceptable, though.
+
 Memory allocated by qemu_memalign or qemu_blockalign must be freed with
 qemu_vfree, since breaking this will cause problems on Win32.