Fix corner case bug in numeric to_char() some more.

The band-aid applied in commit f0bedf3 turns out to still need
some work: it made sure we didn't set Np->last_relevant too small
(to the left of the decimal point), but it didn't prevent setting
it too large (off the end of the partially-converted string).
This could result in fetching data beyond the end of the allocated
space, which with very bad luck could cause a SIGSEGV, though
I don't see any hazard of interesting memory disclosure.

Per bug #17839 from Thiago Nunes.  The bug's pretty ancient,
so back-patch to all supported versions.

Discussion: https://postgr.es/m/17839-aada50db24d7b0da@postgresql.org

src/backend/utils/adt/formatting.c

@@ -5685,13 +5685,20 @@ NUM_processor(FormatNode *node, NUMDesc *Num, char *inout,
 
 			/*
 			 * If any '0' specifiers are present, make sure we don't strip
-			 * those digits.
+			 * those digits.  But don't advance last_relevant beyond the last
+			 * character of the Np->number string, which is a hazard if the
+			 * number got shortened due to precision limitations.
 			 */
 			if (Np->last_relevant && Np->Num->zero_end > Np->out_pre_spaces)
 			{
+				int			last_zero_pos;
 				char	   *last_zero;
 
-				last_zero = Np->number + (Np->Num->zero_end - Np->out_pre_spaces);
+				/* note that Np->number cannot be zero-length here */
+				last_zero_pos = strlen(Np->number) - 1;
+				last_zero_pos = Min(last_zero_pos,
+									Np->Num->zero_end - Np->out_pre_spaces);
+				last_zero = Np->number + last_zero_pos;
 				if (Np->last_relevant < last_zero)
 					Np->last_relevant = last_zero;
 			}

src/test/regress/expected/numeric.out

@@ -1266,6 +1266,12 @@ SELECT '' AS to_char_26, to_char('100'::numeric, 'FM999');
             | 100
 (1 row)
 
+SELECT to_char('12345678901'::float8, 'FM9999999999D9999900000000000000000');
+     to_char     
+-----------------
+ ##########.####
+(1 row)
+
 -- Check parsing of literal text in a format string
 SELECT '' AS to_char_27, to_char('100'::numeric, 'foo999');
  to_char_27 | to_char 

src/test/regress/sql/numeric.sql

@@ -799,6 +799,7 @@ SELECT '' AS to_char_23, to_char(val, '9.999EEEE')				FROM num_data;
 SELECT '' AS to_char_24, to_char('100'::numeric, 'FM999.9');
 SELECT '' AS to_char_25, to_char('100'::numeric, 'FM999.');
 SELECT '' AS to_char_26, to_char('100'::numeric, 'FM999');
+SELECT to_char('12345678901'::float8, 'FM9999999999D9999900000000000000000');
 
 -- Check parsing of literal text in a format string
 SELECT '' AS to_char_27, to_char('100'::numeric, 'foo999');