Guard against no themed rooms or fills being eligible to generate

The themed room code previously assumed that on any given level, at
least one room or fill would resolve as OK to generate there. However,
that's not a great assumption to make, and if it happened to be broken,
the first themed room or fill would arbitrarily be executed, even though
it wasn't eligible. Fix that by setting the initial pick to nil, and
raising an impossible if it's still nil after trying to choose a random
room.

dat/themerms.lua

@@ -936,7 +936,7 @@ function themerooms_generate()
       themerooms[actualrm].contents();
       return
    end
-   local pick = 1;
+   local pick = nil;
    local total_frequency = 0;
    for i = 1, #themerooms do
       if (type(themerooms[i]) ~= "table") then
@@ -957,6 +957,10 @@ function themerooms_generate()
          end
       end
    end
+   if pick == nil then
+      nh.impossible('no eligible themed rooms?')
+      return
+   end
    themerooms[pick].contents();
 end
 
@@ -995,7 +999,7 @@ function themeroom_fill(rm)
       end
       return
    end
-   local pick = 1;
+   local pick = nil;
    local total_frequency = 0;
    for i = 1, #themeroom_fills do
       if (type(themeroom_fills[i]) ~= "table") then
@@ -1016,6 +1020,10 @@ function themeroom_fill(rm)
          end
       end
    end
+   if pick == nil then
+      nh.impossible('no eligible themed room fills?')
+      return
+   end
    themeroom_fills[pick].contents(rm);
 end