Add map.c with shared p_mmap param validation

Forgot to add this file in the previous commit

src/map.c

+/*
+ * Copyright (C) 2009-2012 the libgit2 contributors
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#include <git2/common.h>
+#include "map.h"
+
+int validate_map_args(
+	git_map *out, size_t len, int prot, int flags, int fd, git_off_t offset)
+{
+	GIT_UNUSED(fd);
+	GIT_UNUSED(offset);
+
+	if (out == NULL || len == 0) {
+		errno = EINVAL;
+		giterr_set(GITERR_OS, "Failed to mmap. No map or zero length");
+		return -1;
+	}
+
+	if (!(prot & GIT_PROT_WRITE) && !(prot & GIT_PROT_READ)) {
+		errno = EINVAL;
+		giterr_set(GITERR_OS, "Failed to mmap. Invalid protection parameters");
+		return -1;
+	}
+
+	if (flags & GIT_MAP_FIXED) {
+		errno = EINVAL;
+		giterr_set(GITERR_OS, "Failed to mmap. FIXED not set");
+		return -1;
+	}
+
+	return 0;
+}
+