struct: fix memory leak in buffer.pull()

Do not increase the refcount when returning the pulled buffer contents
as string since the returned value already is the sole reference.
Without this change, pulled buffer contents will be leaked whenever
the `pull()` function is used.

Also ensure that the buffer memory is completely zero initialized when
it is allocated from scratch, the existing logic only cleared the trailing
data area on reallocations but never the head on fresh allocations.

Signed-off-by: Jo-Philipp Wich <[email protected]>

lib/struct.c

@@ -2514,7 +2514,10 @@ grow_buffer(uc_vm_t *vm, void **buf, size_t *bufsz, size_t length)
 			return false;
 		}
 
-		memset(tmp + overhead + old_size - 1, 0, new_size - old_size + 1);
+		if (*buf)
+			memset(tmp + overhead + old_size - 1, 0, new_size - old_size + 1);
+		else
+			memset(tmp, 0, new_size + overhead);
 
 		*buf = tmp;
 		*bufsz = new_size;
@@ -3655,7 +3658,7 @@ uc_fmtbuf_pull(uc_vm_t *vm, size_t nargs)
 	buffer->position = 0;
 	buffer->length = 0;
 
-	return ucv_get(&us->header);
+	return &us->header;
 }