Avoid 'malloc' corner case.

* src/init.c (_gpgrt_realloc): Avoid calling 'malloc(0)'.

Previously, if '_gpgrt_realloc' was called with both A and N being
zero, malloc is invoked with a size of zero. This happens e.g. when
calling '_gpgrt_free' with a NULL pointer, which is supposed to be a
no-op.

Found using the Clang Static Analyzer.

• Signed-off-by: Justus Winter <justus@g10code.com>