Fixes memory leak in memory_buffer_alloc.c debug

Debug symbols were being leaked in memory_buffer_alloc.c
diff --git a/library/memory_buffer_alloc.c b/library/memory_buffer_alloc.c
index b2c775a..545d5a2 100644
--- a/library/memory_buffer_alloc.c
+++ b/library/memory_buffer_alloc.c
@@ -417,6 +417,12 @@
     heap.total_used -= hdr->size;
 #endif
 
+#if defined(MBEDTLS_MEMORY_BACKTRACE)
+    free( hdr->trace );
+    hdr->trace = NULL;
+    hdr->trace_count = 0;
+#endif
+
     // Regroup with block before
     //
     if( hdr->prev != NULL && hdr->prev->alloc == 0 )
@@ -432,9 +438,6 @@
         if( hdr->next != NULL )
             hdr->next->prev = hdr;
 
-#if defined(MBEDTLS_MEMORY_BACKTRACE)
-        free( old->trace );
-#endif
         memset( old, 0, sizeof(memory_header) );
     }
 
@@ -474,9 +477,6 @@
         if( hdr->next != NULL )
             hdr->next->prev = hdr;
 
-#if defined(MBEDTLS_MEMORY_BACKTRACE)
-        free( old->trace );
-#endif
         memset( old, 0, sizeof(memory_header) );
     }
 
@@ -491,11 +491,6 @@
         heap.first_free = hdr;
     }
 
-#if defined(MBEDTLS_MEMORY_BACKTRACE)
-    hdr->trace = NULL;
-    hdr->trace_count = 0;
-#endif
-
     if( ( heap.verify & MBEDTLS_MEMORY_VERIFY_FREE ) && verify_chain() != 0 )
         mbedtls_exit( 1 );
 }