Small fixes to malloc debug.

- Change the printed name from pointer to allocation to be explicit about
  what is wrong.
- Change the signal to be SIGRTMAX - 19 instead of SIGRTMIN. This should
  prevent problems if we have to reserve other real time signals.

Bug: 28218530
Change-Id: Ic7d9c471929264d8e47bafaffc16e099840c9e71
diff --git a/libc/malloc_debug/FreeTrackData.cpp b/libc/malloc_debug/FreeTrackData.cpp
index 682f93d..8e6502e 100644
--- a/libc/malloc_debug/FreeTrackData.cpp
+++ b/libc/malloc_debug/FreeTrackData.cpp
@@ -48,7 +48,7 @@
   uint8_t fill_free_value = debug_->config().fill_free_value;
   for (size_t i = 0; i < header->usable_size; i++) {
     if (pointer[i] != fill_free_value) {
-      error_log("  pointer[%zu] = 0x%02x (expected 0x%02x)", i, pointer[i], fill_free_value);
+      error_log("  allocation[%zu] = 0x%02x (expected 0x%02x)", i, pointer[i], fill_free_value);
     }
   }
   auto back_iter = backtraces_.find(header);