Fix comparisons of pointers with zero to compare with NULL instead.

Per C standard, these are semantically the same thing; but saying NULL
when you mean NULL is good for readability.

Marti Raudsepp, per results of INRIA's Coccinelle.

src/backend/regex/regc_lex.c

@@ -846,7 +846,7 @@ lexescape(struct vars * v)
 			if (ISERR())
 				FAILW(REG_EESCAPE);
 			/* ugly heuristic (first test is "exactly 1 digit?") */
-			if (v->now - save == 0 || ((int) c > 0 && (int) c <= v->nsubexp))
+			if (v->now == save || ((int) c > 0 && (int) c <= v->nsubexp))
 			{
 				NOTE(REG_UBACKREF);
 				RETV(BACKREF, (chr) c);

src/backend/utils/adt/tsrank.c

@@ -395,7 +395,7 @@ getWeights(ArrayType *win)
 	int			i;
 	float4	   *arrdata;
 
-	if (win == 0)
+	if (win == NULL)
 		return weights;
 
 	if (ARR_NDIM(win) != 1)

src/backend/utils/fmgr/dfmgr.c

@@ -616,7 +616,7 @@ find_in_dynamic_libpath(const char *basename)
 					(errcode(ERRCODE_INVALID_NAME),
 					 errmsg("zero-length component in parameter \"dynamic_library_path\"")));
 
-		if (piece == 0)
+		if (piece == NULL)
 			len = strlen(p);
 		else
 			len = piece - p;