Throw an error instead of assert-crashing on eval stack underflow.

eval.c

@@ -479,9 +479,10 @@ static bool do_one_inside(int begin, int end)
         }
         else
         {
-            int next;
-            assert(loc >= 0);
-            next = stacks[loc].pos;
+            if (loc < 0)
+                return false;
+
+            int next = stacks[loc].pos;
             if (ploc == -1 || stacks[next].pos == 0 || stacks[next].prio != PRIO_LITERAL)
                 return false;
             if (stacks[ploc].prio != PRIO_LITERAL)

tests/data/badmath.in

@@ -0,0 +1,4 @@
+#math x ()
+#math x 1/
+#math x 1/0
+#math x ))

tests/data/badmath.out

@@ -0,0 +1,4 @@
+#Invalid expression to evaluate in {()}
+#Invalid expression to evaluate in {1/}
+#Error: Division by zero.
+#Unmatched parentheses error in {))}.