alloc: don't overwrite allocator during init if set

If the allocator has been set before we the library is initialised, we would
replace that setting with the standard allocator contrary to the user's wishes.

src/alloc.c

@@ -17,6 +17,13 @@ git_allocator git__allocator;
 
 int git_allocator_global_init(void)
 {
+	/*
+	 * We don't want to overwrite any allocator which has been set before
+	 * the init function is called.
+	 */
+	if (git__allocator.gmalloc != NULL)
+		return 0;
+
 #if defined(GIT_MSVC_CRTDBG)
 	return git_win32_crtdbg_init_allocator(&git__allocator);
 #else