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

(cherry picked from commit ea26b33248663a836d2f0719cb4e46967b648fe2)

Change-Id: I707d5200c1804bdbf45fc46de0f2565016c48a82
diff --git a/libc/malloc_debug/GuardData.cpp b/libc/malloc_debug/GuardData.cpp
index c70e8f1..97b16ff 100644
--- a/libc/malloc_debug/GuardData.cpp
+++ b/libc/malloc_debug/GuardData.cpp
@@ -58,7 +58,7 @@
   const uint8_t* real = reinterpret_cast<const uint8_t*>(data);
   for (size_t i = 0; i < cmp_mem_.size(); i++, pointer_idx++) {
     if (real[i] != expected[i]) {
-      error_log("  pointer[%d] = 0x%02x (expected 0x%02x)", pointer_idx, real[i], expected[i]);
+      error_log("  allocation[%d] = 0x%02x (expected 0x%02x)", pointer_idx, real[i], expected[i]);
     }
   }