[PATCH 1/2] tree-diff: fix leak when not HAVE_ALLOCA

b8ba412bf7 (tree-diff: avoid alloca for large allocations, 2016-06-07)
adds a way to route some bigger allocations out of the stack and free
them through the addition of two conveniently named macros, but leaves
the calls to free the xalloca part, which could be also in the heap,
if the system doesn't HAVE_ALLOCA (ex: macOS).

Add the missing free call, and while at it, change the expression to
match in both macros for easy of readability.

This avoids a leak reported by LSAN as while running t0000 but that
wouldn't fail the test (which will be fixed next) :

  SUMMARY: LeakSanitizer: 1034 byte(s) leaked in 15 allocation(s).

Signed-off-by: Carlo Marcelo Arenas Belón <redacted>
---
 tree-diff.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tree-diff.c b/tree-diff.c
index 1572615bd9..437c98a70e 100644
--- a/tree-diff.c
+++ b/tree-diff.c

@@ -21,7 +21,9 @@
 		ALLOC_ARRAY((x), nr); \
 } while(0)
 #define FAST_ARRAY_FREE(x, nr) do { \
-	if ((nr) > 2) \
+	if ((nr) <= 2) \
+		xalloca_free((x)); \
+	else \
 		free((x)); \
 } while(0)
 

-- 
2.33.0.481.g26d3bed244