Disable leak checking for mksh; it's way too leaky.

When each shell leaks ~240 allocations, you can't see the leaks from
the program you ran with "adb shell".

Bug: 7291287
Change-Id: Ib8780db72ba0114ebdb24768537da74bbb61f354

libc/bionic/malloc_debug_common.cpp

@@ -376,6 +376,11 @@ static void malloc_init_impl() {
         }
     }
 
+    // mksh is way too leaky. http://b/7291287.
+    if (debug_level >= 10 && strcmp(__progname, "sh") == 0) {
+        return;
+    }
+
     // Choose the appropriate .so for the requested debug level.
     switch (debug_level) {
         case 1: