Fix Issue 17847 - Properly sanitize seeds for Park–Miller engines

As in XorshiftEngine, sanitizing the seed is more useful than throwing
an exception. The original check was broken because it checked the seed
before taking the modulus. Making this change allows unpredictableSeed
to be marked nothrow and @nogc.

std/random.d

@@ -362,23 +362,23 @@ The parameters of this distribution. The random number is $(D_PARAM x
 Constructs a $(D_PARAM LinearCongruentialEngine) generator seeded with
 $(D x0).
  */
-    this(UIntType x0) @safe pure
+    this(UIntType x0) @safe pure nothrow @nogc
     {
         seed(x0);
     }
 
 /**
    (Re)seeds the generator.
 */
-    void seed(UIntType x0 = 1) @safe pure
+    void seed(UIntType x0 = 1) @safe pure nothrow @nogc
     {
+        _x = modulus ? (x0 % modulus) : x0;
         static if (c == 0)
         {
-            import std.exception : enforce;
-            enforce(x0, "Invalid (zero) seed for "
-                    ~ LinearCongruentialEngine.stringof);
+            //Necessary to prevent generator from outputting an endless series of zeroes.
+            if (_x == 0)
+                _x = max;
         }
-        _x = modulus ? (x0 % modulus) : x0;
         popFront();
     }
 
@@ -537,6 +537,14 @@ alias MinstdRand = LinearCongruentialEngine!(uint, 48_271, 0, 2_147_483_647);
     }
 }
 
+@safe unittest
+{
+    auto rnd0 = MinstdRand0(MinstdRand0.modulus);
+    auto n = rnd0.front;
+    rnd0.popFront();
+    assert(n != rnd0.front);
+}
+
 /**
 The $(LINK2 https://en.wikipedia.org/wiki/Mersenne_Twister, Mersenne Twister) generator.
  */
@@ -1291,7 +1299,7 @@ random number sequences every run.
 Returns:
 A single unsigned integer seed value, different on each successive call
 */
-@property uint unpredictableSeed() @trusted
+@property uint unpredictableSeed() @trusted nothrow @nogc
 {
     import core.thread : Thread, getpid, MonoTime;
     static bool seeded;