jscoverage: fix build with gcc6 (#29423)

pkgs/development/tools/misc/jscoverage/default.nix

@@ -8,6 +8,10 @@ stdenv.mkDerivation rec {
     sha256 = "c45f051cec18c10352f15f9844f47e37e8d121d5fd16680e2dd0f3b4420eb7f4";
   };
 
+  patches = [
+    ./jsfalse_to_null.patch
+  ];
+
   buildInputs = [ autoconf makedepend perl python unzip zip ];
 
   # It works without MOZ_FIX_LINK_PATHS, circumventing an impurity

pkgs/development/tools/misc/jscoverage/jsfalse_to_null.patch

@@ -0,0 +1,26 @@
+diff --git i/js/jsstr.cpp w/js/jsstr.cpp
+index e662bf1..b18597e 100644
+--- i/js/jsstr.cpp
++++ w/js/jsstr.cpp
+@@ -3040,7 +3040,7 @@ js_InitStringClass(JSContext *cx, JSObject *obj)
+                                  JSVAL_VOID, NULL, NULL,
+                                  JSPROP_READONLY | JSPROP_PERMANENT | JSPROP_SHARED, 0, 0,
+                                  NULL)) {
+-        return JS_FALSE;
++        return NULL;
+     }
+ 
+     return proto;
+diff --git i/js/jsxml.cpp w/js/jsxml.cpp
+index 5621ede..e00c97d 100644
+--- i/js/jsxml.cpp
++++ w/js/jsxml.cpp
+@@ -303,7 +303,7 @@ NewXMLNamespace(JSContext *cx, JSString *prefix, JSString *uri, JSBool declared)
+ 
+     obj = js_NewObject(cx, &js_NamespaceClass.base, NULL, NULL);
+     if (!obj)
+-        return JS_FALSE;
++        return NULL;
+     JS_ASSERT(JSVAL_IS_VOID(obj->fslots[JSSLOT_PREFIX]));
+     JS_ASSERT(JSVAL_IS_VOID(obj->fslots[JSSLOT_URI]));
+     JS_ASSERT(JSVAL_IS_VOID(obj->fslots[JSSLOT_DECLARED]));