tools/dat2c: Fix a buffer overlow

When prefix is set and datafile_name is not set then
strlen(prefix) + 6 bytes are necessary, not strlen(prefix) 5,
because then we get: "$(prefix)_data\0" which requires 6 bytes
extra for "_data\0".

This fixes dat2c crashing like this:

 dat2c mbdata.dat -o mbdata.c -h mbdata.h -p mb -g
 *** buffer overflow detected ***: terminated
 make: *** [Makefile:45: mbdata.c] Aborted (core dumped)

When compiled with a recent gcc version + a high FORTIFY_SOURCE setting.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2225996
diff -up allegro-4.4.3.1/tools/dat2c.c~ allegro-4.4.3.1/tools/dat2c.c
--- allegro-4.4.3.1/tools/dat2c.c~	2019-03-04 02:30:11.000000000 +0100
+++ allegro-4.4.3.1/tools/dat2c.c	2023-07-26 20:12:52.250765448 +0200
@@ -1641,9 +1641,10 @@ int do_conversion(struct dat2c *dat2c)
    int result = 0;
    char *prefixed_name = 0;
 
-   prefixed_name = malloc(5 +
+   /* 2 for '_' + '\0' */
+   prefixed_name = malloc(2 +
       (dat2c->prefix ? (signed)strlen(dat2c->prefix) : 0) +
-      (dat2c->datafile_name ? (signed)strlen(dat2c->datafile_name) : 0));
+      (dat2c->datafile_name ? (signed)strlen(dat2c->datafile_name) : 4));
    if (!prefixed_name)
       out_of_memory();
    sprintf(prefixed_name, "%s%s%s",