Fix '.echo >0' crash in first pass

When run_first_pass_line / run_first_pass_line_sec changes the ptr
to NULL, run_first_pass does not check for this and attempts to
dereference the NULL ptr, causing a crash. Adding a NULL check and
breaking out of the loops fixes this!

Fixes one of the fuzzed lines in #43.

pass_one.cpp

@@ -148,6 +148,9 @@ void run_first_pass (char *ptr) {
 				do_listing_for_line (ptr);
 
 			line_num++;
+		} else {
+			// Nothing more to read, return
+			break;
 		}
 	}
 }