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Store a size field inside the DSF type.

This permits bounds-checking of all inputs to dsf_canonify and
dsf_merge, so that any out-of-range values will provoke assertion
failure instead of undefined behaviour.

--- a/dsf.c

+++ b/dsf.c

@@ -10,6 +10,7 @@

 #include "puzzles.h"

 struct DSF {

+    int size;

     int *p;

 };

@@ -86,6 +87,7 @@

 DSF *snew_dsf(int size)

 {

     DSF *ret = snew(DSF);

+    ret->size = size;

     ret->p = snewn(size, int);

     dsf_init(ret, size);

@@ -125,7 +127,7 @@

 /*    fprintf(stderr, "dsf = %p\n", dsf); */

 /*    fprintf(stderr, "Canonify %2d\n", index); */

-    assert(index >= 0);

+    assert(0 <= index && index < dsf->size && "Overrun in edsf_canonify");

     /* Find the index of the canonical element of the 'equivalence class' of

      * which start_index is a member, and figure out whether start_index is the

@@ -162,6 +164,9 @@

 void edsf_merge(DSF *dsf, int v1, int v2, bool inverse)

 {

     bool i1, i2;

+

+    assert(0 <= v1 && v1 < dsf->size && "Overrun in edsf_merge");

+    assert(0 <= v2 && v2 < dsf->size && "Overrun in edsf_merge");

 /*    fprintf(stderr, "dsf = %p\n", dsf); */

 /*    fprintf(stderr, "Merge [%2d,%2d], %d\n", v1, v2, inverse); */