xdiff: NUL-terminate buffers read by read_mmfile()

Since an mmfile_t is a ptr/len pair, our read_mmfile() allocates exactly
the number of bytes we claim to store. But in many other places in Git,
we add an extra NUL "just in case", which can help avoid read overruns
due to off-by-ones or the use of string functions.

I don't know of any path that would benefit from this, but I noticed it
while converting ll_ext_merge() to use read_mmfile(), since its original
code did add a NUL byte (even though I cannot find any case where it
would have mattered). Let's add the same defensive NUL in read_mmfile()
by using xmallocz() instead of xmalloc().

Note that the matching read_mmblob() doesn't need the same treatment.
Its buffers already have a NUL from the object-reading code (which uses
the same defensive trick).

As a bonus, we can get rid of the hack in read_mmfile() to handle empty
files by allocating a single byte.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff-interface.c

@@ -166,7 +166,7 @@ int read_mmfile(mmfile_t *ptr, const char *filename)
 	if (!(f = fopen(filename, "rb")))
 		return error_errno("Could not open %s", filename);
 	sz = xsize_t(st.st_size);
-	ptr->ptr = xmalloc(sz ? sz : 1);
+	ptr->ptr = xmallocz(sz);
 	if (sz && fread(ptr->ptr, sz, 1, f) != 1) {
 		FREE_AND_NULL(ptr->ptr);
 		fclose(f);