libc: add message for corrupted allocation on 'free'

In case the meta data (or more) got zeroed upon 'free', print error
message. The offset in the meta data can never be zero. This does not
help on other memory corruptions, but at least gives a hint in the too
much zeroed out case.

issue #4675

repos/libports/src/lib/libc/malloc.cc

@@ -221,6 +221,10 @@ class Libc::Malloc
 
 			void *alloc_addr = (void *)((addr_t)ptr - md->offset);
 
+			if (md->offset == 0)
+				error("libc free: meta-data offset is 0 for address: ", ptr,
+				      " - corrupted allocation");
+
 			if (msb > SLAB_STOP) {
 				_backing_store.free(alloc_addr, real_size);
 			} else {