Fixed crash caused by 0-length lines in UDMF

When line was removed during processing bogus entries remained in sidedefs array

src/p_udmf.cpp

@@ -1934,10 +1934,17 @@ class UDMFParser : public UDMFParserBase
 			P_AdjustLine(&lines[line]);
 			P_FinishLoadingLineDef(&lines[line], tempalpha[0]);
 		}
-		assert((unsigned)side <= level.sides.Size());
-		if ((unsigned)side > level.sides.Size())
+
+		const int sideDelta = level.sides.Size() - side;
+		assert(sideDelta >= 0);
+
+		if (sideDelta < 0)
+		{
+			Printf("Map had %d invalid side references\n", abs(sideDelta));
+		}
+		else if (sideDelta > 0)
 		{
-			Printf("Map had %d invalid side references\n", (int)level.sides.Size() - side);
+			level.sides.Resize(side);
 		}
 	}