Fix ICE when parsing an invalid terse function initializer

Closes #1006

Author: hsutter

source/build.info

@@ -1 +1 @@
-"9712:0834"
+"9712:0850"

source/parse.h

@@ -8575,7 +8575,9 @@ class parser
         {
             n->equal_sign = curr().position();
             n->initializer = statement(/*ignore semicolon_required*/ false, n->equal_sign);
-            assert( n->initializer && "ICE: should have already validated that there's a valid expression-statement here" );
+            if (!n->initializer) {
+                return {};
+            }
         }
 
         else