Fix some null deferences in the test suite (#21617)

Author: thewilsonator

compiler/test/compilable/mixintype.d

@@ -4,9 +4,9 @@ alias Lint = mixin("Int");
 
 int test1(mixin("int")* p)
 {
-    mixin("int")[] a;
-    mixin("int[]") b;
-    mixin("int[] c;");
+    mixin("int")[] a = [1];
+    mixin("int[]") b = [1];
+    mixin("int[] c = [1];");
     mixin("*p = c[0];");
     *p = mixin("c[0]");
     return *p + a[0] + b[0] + c[0];

compiler/test/compilable/parens_inc.d

@@ -6,7 +6,8 @@ void main(){
     *(x)++=0;
     (*(x)--)=0;
     (*x++)=0; // ok
-    int*[] z;
+    int a = 1, b = 2;
+    int*[] z = [&a, &b];
     *(z[0])++=0; //ok
     (y[0])--;
     *x++=0;

compiler/test/compilable/test17146.d

@@ -6,7 +6,7 @@ struct S { int[] a; int b; }
 
 void foo()
 {
-    S[] s;
+    S[] s = [S([],0)];
     if (s[$-1] == S.init) {}
 }
 

compiler/test/fail_compilation/test18282.d

@@ -15,7 +15,7 @@ fail_compilation/test18282.d(53): Error: escaping a reference to local variable
 
 string* f() @safe
 {
-    scope string*[] ls;
+    scope string*[] ls = [null];
     return ls[0];
 }
 

compiler/test/runnable/constfold.d

@@ -418,7 +418,7 @@ class C8939regression
         string[2] str;
         refValue(str[n1]);
 
-        int[] da;
+        int[] da = [1];
         refValue(da[n2]);
 
         int n; int* p = &n;