patch 9.0.0013: reproducing memory access errors can be difficult

Problem:    Reproducing memory access errors can be difficult.
Solution:   When testing, copy each line to allocated memory, so that valgrind
            can detect accessing memory before and/or after it.  Fix uncovered
            problems.

src/normal.c

@@ -5120,6 +5120,8 @@ n_swapchar(cmdarg_T *cap)
 			count = (int)STRLEN(ptr) - pos.col;
 			netbeans_removed(curbuf, pos.lnum, pos.col,
 								 (long)count);
+			// line may have been flushed, get it again
+			ptr = ml_get(pos.lnum);
 			netbeans_inserted(curbuf, pos.lnum, pos.col,
 							&ptr[pos.col], count);
 		    }