merge-file: handle empty files gracefully

Earlier, it would error out while trying to read and/or writing them.
Now, calling merge-file with empty files is neither interesting nor
useful, but it is a bug that needed fixing.

Noticed by Clemens Buchacher.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin-merge-file.c

@@ -57,7 +57,8 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 
 		if (!f)
 			ret = error("Could not open %s for writing", filename);
-		else if (fwrite(result.ptr, result.size, 1, f) != 1)
+		else if (result.size &&
+			 fwrite(result.ptr, result.size, 1, f) != 1)
 			ret = error("Could not write to %s", filename);
 		else if (fclose(f))
 			ret = error("Could not close %s", filename);

xdiff-interface.c

@@ -152,8 +152,8 @@ int read_mmfile(mmfile_t *ptr, const char *filename)
 	if ((f = fopen(filename, "rb")) == NULL)
 		return error("Could not open %s", filename);
 	sz = xsize_t(st.st_size);
-	ptr->ptr = xmalloc(sz);
+	ptr->ptr = xmalloc(sz ? sz : 1);
-	if (fread(ptr->ptr, sz, 1, f) != 1)
+	if (sz && fread(ptr->ptr, sz, 1, f) != 1)
 		return error("Could not read %s", filename);
 	fclose(f);
 	ptr->size = sz;