Suppress new "may be used uninitialized" warning.

Various buildfarm members, having compilers like gcc 8.5 and 6.3, fail
to deduce that text_substring() variable "E" is initialized if
slice_size!=-1.  This suppression approach quiets gcc 8.5; I did not
reproduce the warning elsewhere.  Back-patch to v14, like commit
9f4fd119b2cbb9a41ec0c19a8d6ec9b59b92c125.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1157953.1771266105@sss.pgh.pa.us
Backpatch-through: 14

Author: nmisch

src/backend/utils/adt/varlena.c

@@ -975,14 +975,14 @@ text_substring(Datum str, int32 start, int32 length, bool length_not_specified)
 
 		if (length_not_specified)	/* special case - get length to end of
 									 * string */
-			slice_size = L1 = -1;
+			E = slice_size = L1 = -1;
 		else if (length < 0)
 		{
 			/* SQL99 says to throw an error for E < S, i.e., negative length */
 			ereport(ERROR,
 					(errcode(ERRCODE_SUBSTRING_ERROR),
 					 errmsg("negative substring length not allowed")));
-			slice_size = L1 = -1;	/* silence stupider compilers */
+			E = slice_size = L1 = -1;	/* silence stupider compilers */
 		}
 		else if (pg_add_s32_overflow(S, length, &E))
 		{