tcc_mallocz: clear with memset only if nonzero size
tcc_mallocz: clear with memset only if nonzero size

Even if non-NULL, dereferencing the pointer from malloc(0)
or passing it to memset() may invoke undefined behavior.

mingodad/tinycc,mirror/tinycc,mingodad/tinycc,mingodad/tinycc,mingodad/tinycc,mirror/tinycc,mirror/tinycc,mirror/tinycc