Make printf("%s", NULL) print "(null)" instead of crashing.

We previously took a hard-line attitude that callers should never print
a null string pointer, and doing so is worthy of an assertion failure
or crash.  However, we've long since flushed out any easy-to-find bugs
of that nature.  What remains is a lot of code that perhaps could fail
that way in hard-to-reach corner cases.  For example, in something as
simple as
    ereport(ERROR,
            (errcode(ERRCODE_UNDEFINED_OBJECT),
             errmsg("constraint \"%s\" for table \"%s\" does not exist",
                    conname, get_rel_name(relid))));
one must wonder whether it's completely guaranteed that get_rel_name
cannot return NULL in this context.  If such a situation did occur,
the existing policy converts what might be a pretty minor bug into
a server crash condition.  This is not good for robustness.

Hence, let's follow the lead of glibc and print "(null)" instead
of failing.  We should, of course, still consider it a bug if that
behavior is reachable in ordinary use; but crashing seems less
desirable than not crashing.

This fix works across-the-board in v12 and up, where we always use
src/port/snprintf.c.  Before that, on most platforms we're at the mercy
of the local libc, but it appears that Solaris 10 is the only supported
platform where we'd still get a crash.  Most other platforms such as
*BSD, macOS, and Solaris 11 have adopted glibc's behavior at some
point.  (AIX and HPUX just print "" not "(null)", but that's close
enough.)  I've not checked what Windows' native printf would do, but
it doesn't matter because we've long used snprintf.c on that platform.

In v12 and up, also const-ify related code so that we're not casting
away const on the constant string.  This is just neatnik-ism, since
next to no compilers will warn about that.

Discussion: https://postgr.es/m/17098-b960f3616c861f83@postgresql.org

src/port/snprintf.c

@@ -320,7 +320,7 @@ static bool find_arguments(const char *format, va_list args,
 						   PrintfArgValue *argvalues);
 static void fmtstr(const char *value, int leftjust, int minlen, int maxwidth,
 				   int pointflag, PrintfTarget *target);
-static void fmtptr(void *value, PrintfTarget *target);
+static void fmtptr(const void *value, PrintfTarget *target);
 static void fmtint(long long value, char type, int forcesign,
 				   int leftjust, int minlen, int zpad, int precision, int pointflag,
 				   PrintfTarget *target);
@@ -394,7 +394,7 @@ dopr(PrintfTarget *target, const char *format, va_list args)
 	int			cvalue;
 	long long	numvalue;
 	double		fvalue;
-	char	   *strvalue;
+	const char *strvalue;
 	PrintfArgValue argvalues[PG_NL_ARGMAX + 1];
 
 	/*
@@ -439,7 +439,8 @@ dopr(PrintfTarget *target, const char *format, va_list args)
 		{
 			format++;
 			strvalue = va_arg(args, char *);
-			Assert(strvalue != NULL);
+			if (strvalue == NULL)
+				strvalue = "(null)";
 			dostr(strvalue, strlen(strvalue), target);
 			if (target->failed)
 				break;
@@ -670,8 +671,9 @@ nextch2:
 					strvalue = argvalues[fmtpos].cptr;
 				else
 					strvalue = va_arg(args, char *);
-				/* Whine if someone tries to print a NULL string */
-				Assert(strvalue != NULL);
+				/* If string is NULL, silently substitute "(null)" */
+				if (strvalue == NULL)
+					strvalue = "(null)";
 				fmtstr(strvalue, leftjust, fieldwidth, precision, pointflag,
 					   target);
 				break;
@@ -681,7 +683,7 @@ nextch2:
 					strvalue = argvalues[fmtpos].cptr;
 				else
 					strvalue = va_arg(args, char *);
-				fmtptr((void *) strvalue, target);
+				fmtptr((const void *) strvalue, target);
 				break;
 			case 'e':
 			case 'E':
@@ -995,7 +997,7 @@ fmtstr(const char *value, int leftjust, int minlen, int maxwidth,
 }
 
 static void
-fmtptr(void *value, PrintfTarget *target)
+fmtptr(const void *value, PrintfTarget *target)
 {
 	int			vallen;
 	char		convert[64];