BUG#20622: Fix one-byte buffer overrun in IM directory string handling.

The problem was a call to convert_dirname() with a destination buffer
that did not have room for the trailing slash added by that function.
This could cause the instance manager to crash in some cases.

mysys/mf_dirname.c

@@ -72,7 +72,9 @@ uint dirname_part(my_string to, const char *name)
 
   SYNPOSIS
     convert_dirname()
-    to				Store result here
+    to				Store result here. Must be at least of size
+    				min(FN_REFLEN, strlen(from) + 1) to make room
+    				for adding FN_LIBCHAR at the end.
     from			Original filename
     from_end			Pointer at end of filename (normally end \0)
 

server-tools/instance-manager/instance_options.cc

@@ -391,8 +391,13 @@ int Instance_options::complete_initialization(const char *default_path,
   const char *tmp;
   char *end;
 
-  if (!mysqld_path && !(mysqld_path= strdup_root(&alloc, default_path)))
-    goto err;
+  if (!mysqld_path)
+  {
+    // Need one extra byte, as convert_dirname() adds a slash at the end.
+    if (!(mysqld_path= alloc_root(&alloc, strlen(default_path) + 2)))
+      goto err;
+    strcpy((char *)mysqld_path, default_path);
+  }
 
   // it's safe to cast this to char* since this is a buffer we are allocating
   end= convert_dirname((char*)mysqld_path, mysqld_path, NullS);