compat: make sure git_mmap is not expected to write

in f48000f ("Yank writing-back support from gitfakemmap.", 2005-10-08)
support for writting back changes was removed but the specific prot
flag that would be used was not checked for

Signed-off-by: Carlo Marcelo Arenas Belón <[email protected]>
Acked-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: carenas

compat/mmap.c

@@ -4,7 +4,7 @@ void *git_mmap(void *start, size_t length, int prot, int flags, int fd, off_t of
 {
 	size_t n = 0;
 
-	if (start != NULL || !(flags & MAP_PRIVATE))
+	if (start != NULL || flags != MAP_PRIVATE || prot != PROT_READ)
 		die("Invalid usage of mmap when built with NO_MMAP");
 
 	start = xmalloc(length);