Fix bad free when limit exceeded.

If the android limit code is enabled and a realloc would cross the
limit, the old code would free the original pointer. However, this
is invalid and the original pointer should not be changed.

No longer free the pointer in this case and add a test for this
case.

Bug: 501541998
hashtag:b/501541998

Test: All unit tests pass.
Test: Verify that reverting back to old behavior, new test fails.
(cherry picked from commit d00918bfbbe06d4a0841d52aabe6dea3a2432787)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:94db0b5e8c2fcc0d63f5664c1465eaba046b9cae
Merged-In: I52ccd62f4f513aa55989fb7dda816d13e3171120
Change-Id: I52ccd62f4f513aa55989fb7dda816d13e3171120

libc/bionic/malloc_limit.cpp

diff --git a/libc/bionic/malloc_limit.cpp b/libc/bionic/malloc_limit.cpp
index 85ddc79..ea771b6 100644
--- a/libc/bionic/malloc_limit.cpp
+++ b/libc/bionic/malloc_limit.cpp
@@ -201,8 +201,7 @@
   if (bytes > old_usable_size && !CheckLimit(bytes - old_usable_size)) {
     warning_log("malloc_limit: realloc(%p, %zu) exceeds limit %" PRId64, old_mem, bytes,
                 gAllocLimit);
-    // Free the old pointer.
-    LimitFree(old_mem);
+    // Do not free the old pointer, it's still valid.
     return nullptr;
   }
 

tests/malloc_test.cpp

diff --git a/tests/malloc_test.cpp b/tests/malloc_test.cpp
index 46f2342..cb3f387 100644
--- a/tests/malloc_test.cpp
+++ b/tests/malloc_test.cpp
@@ -818,8 +818,10 @@
   memory = realloc(memory, 80 * 1024 * 1024);
   ASSERT_TRUE(memory != nullptr);
   // Now push past limit.
-  memory = realloc(memory, 130 * 1024 * 1024);
-  ASSERT_TRUE(memory == nullptr);
+  void* new_memory = realloc(memory, 130 * 1024 * 1024);
+  ASSERT_TRUE(new_memory == nullptr);
+  // The original pointer should still be valid, so free it.
+  free(memory);
 
   VerifyMaxPointers(max_pointers);
 #else