patch 8.0.1495: having 'pumwidth' default to zero has no merit

Problem:    Having 'pumwidth' default to zero has no merit.
Solution:   Make the default 15, as the actual default value.
diff --git a/src/option.c b/src/option.c
index 5e215d5..e54ba4e 100644
--- a/src/option.c
+++ b/src/option.c
@@ -2245,7 +2245,7 @@
 #else
 			    (char_u *)NULL, PV_NONE,
 #endif
-			    {(char_u *)0L, (char_u *)0L} SCRIPTID_INIT},
+			    {(char_u *)15L, (char_u *)15L} SCRIPTID_INIT},
     {"pythonthreedll",  NULL,   P_STRING|P_EXPAND|P_VI_DEF|P_SECURE,
 #if defined(DYNAMIC_PYTHON3)
 			    (char_u *)&p_py3dll, PV_NONE,