Avoid dereferencing an undefined pointer in DecodeInterval().

Commit e39f99046 moved some code up closer to the start of
DecodeInterval(), without noticing that it had been implicitly
relying on previous checks to reject the case of empty input.
Given empty input, we'd now dereference a pointer that hadn't been
set, possibly leading to a core dump.  (But if we fail to provoke
a SIGSEGV, nothing bad happens, and the expected syntax error is
thrown a bit later.)

Per bug #17788 from Alexander Lakhin.  Back-patch to v15 where
the fault was introduced.

Discussion: https://postgr.es/m/17788-dabac9f98f7eafd5@postgresql.org

src/backend/utils/adt/datetime.c

@@ -3376,7 +3376,7 @@ DecodeInterval(char **field, int *ftype, int nf, int range,
 	 * to dump in postgres style, not SQL style.)
 	 *----------
 	 */
-	if (IntervalStyle == INTSTYLE_SQL_STANDARD && *field[0] == '-')
+	if (IntervalStyle == INTSTYLE_SQL_STANDARD && nf > 0 && *field[0] == '-')
 	{
 		force_negative = true;
 		/* Check for additional explicit signs */

src/test/regress/expected/interval.out

@@ -888,6 +888,11 @@ SELECT  interval '-23 hours 45 min 12.34 sec',
  -23:45:12.34 | -1 23:45:12.34 | -1-2 -1 -23:45:12.34 | -0-10 +1 +23:45:12.34
 (1 row)
 
+-- edge case for sign-matching rules
+SELECT  interval '';  -- error
+ERROR:  invalid input syntax for type interval: ""
+LINE 1: SELECT  interval '';
+                         ^
 -- test outputting iso8601 intervals
 SET IntervalStyle to iso_8601;
 select  interval '0'                                AS "zero",

src/test/regress/sql/interval.sql

@@ -284,6 +284,9 @@ SELECT  interval '-23 hours 45 min 12.34 sec',
         interval '-1 year 2 months 1 day 23 hours 45 min 12.34 sec',
         interval '-1 year 2 months 1 day 23 hours 45 min +12.34 sec';
 
+-- edge case for sign-matching rules
+SELECT  interval '';  -- error
+
 -- test outputting iso8601 intervals
 SET IntervalStyle to iso_8601;
 select  interval '0'                                AS "zero",