Fix memory leak on unset of associative array (#64)

Associative arrays weren't being properly freed from memory, which
was causing a memory leak.

This commit incorporates a patch and reproducer/regress test from:
https://www.mail-archive.com/ast-users@lists.research.att.com/msg01016.html

src/cmd/ksh93/sh/name.c:
- Properly free associative arrays from memory in nv_delete().

src/cmd/ksh93/tests/leaks.sh:
- Add regression test.

NEWS

@@ -8,6 +8,8 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - 'notty' is now written to the ksh auditing file instead of '(null)' if
   the user's tty could not be determined.
 
+- Unsetting an associative array no longer causes a memory leak to occur.
+
 2020-07-05:
 
 - In UTF-8 locales, fix corruption of the shell's internal string quoting

src/cmd/ksh93/sh/name.c

@@ -1298,7 +1298,18 @@ void nv_delete(Namval_t* np, Dt_t *root, int flags)
 		if(dtdelete(root,np))
 		{
 			if(!(flags&NV_NOFREE) && ((flags&NV_FUNCTION) || !nv_subsaved(np)))
+			{
+				Namarr_t *ap;
+				if(nv_isarray(np) && np->nvfun && (ap=nv_arrayptr(np)) && array_assoc(ap))
+				{
+					/* free associative array from memory */
+					while(nv_associative(np,0,NV_ANEXT))
+						nv_associative(np,0,NV_ADELETE);
+					nv_associative(np,0,NV_AFREE);
+					free((void*)np->nvfun);
+				}
 				free((void*)np);
+			}
 		}
 #if 0
 		else

src/cmd/ksh93/tests/leaks.sh

@@ -101,4 +101,25 @@ done
 after=$(getmem)
 (( after > before )) && err_exit "memory leak with read -C when using <<< (leaked $((after - before)) $unit)"
 
+# ======
+# Unsetting an associative array shouldn't cause a memory leak
+# See https://www.mail-archive.com/ast-users@lists.research.att.com/msg01016.html
+typeset -A stuff
+before=$(getmem)
+for (( i=0; i<1000; i++ ))
+do
+	unset stuff[xyz]
+	typeset -A stuff[xyz]
+	stuff[xyz][elem0]="data0"
+	stuff[xyz][elem1]="data1"
+	stuff[xyz][elem2]="data2"
+	stuff[xyz][elem3]="data3"
+	stuff[xyz][elem4]="data4"
+done
+unset stuff
+after=$(getmem)
+(( after > before )) && err_exit 'unset of associative array causes memory leak' \
+	"(leaked $((after - before)) $unit)"
+
+# ======
 exit $((Errors<125?Errors:125))