Add some extra assert() statements to silence a few clang warnings.

FossilOrigin-Name: 1c63e9515b5f5cb9169928b108d002dcd18ad4fd
diff --git a/src/expr.c b/src/expr.c
index 5c5297c..e857193 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -1279,6 +1279,7 @@
     case TK_UMINUS: {
       int v;
       if( sqlite3ExprIsInteger(p->pLeft, &v) ){
+        assert( v!=-2147483648 );
         *pValue = -v;
         rc = 1;
       }