Fix incorrect strncmp usage when comparing access method names.

Previously, the third argument to strncmp() was a boolean expression,
which resulted in a -Wmemsize-comparison warning and incorrect behavior.
Since the lengths were already checked to be equal, strcmp() is sufficient
and more appropriate here.

Author: yjhjstz

src/backend/parser/gram.y

@@ -3850,7 +3850,7 @@ alter_table_cmd:
 					if (witham) 
 					{
 						if (strlen(witham) != strlen(n->name) || 
-							strncmp(n->name, witham, strlen(n->name) != 0))
+							strcmp(n->name, witham) != 0)
 							ereport(ERROR,
 									(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 									 errmsg("ACCESS METHOD is specified as \"%s\" but "