Don't leak malloc'd strings when a GUC setting is rejected.

Because guc.c prefers to keep all its string values in malloc'd
not palloc'd storage, it has to be more careful than usual to
avoid leaks.  Error exits out of string GUC hook checks failed
to clear the proposed value string, and error exits out of
ProcessGUCArray() failed to clear the malloc'd results of
ParseLongOption().

Found via valgrind testing.
This problem is ancient, so back-patch to all supported branches.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us

src/backend/utils/misc/guc.c

@@ -9770,6 +9770,8 @@ ProcessGUCArray(ArrayType *array,
 		char	   *s;
 		char	   *name;
 		char	   *value;
+		char	   *namecopy;
+		char	   *valuecopy;
 
 		d = array_ref(array, 1, &i,
 					  -1 /* varlenarray */ ,
@@ -9794,13 +9796,18 @@ ProcessGUCArray(ArrayType *array,
 			continue;
 		}
 
-		(void) set_config_option(name, value,
+		/* free malloc'd strings immediately to avoid leak upon error */
+		namecopy = pstrdup(name);
+		free(name);
+		valuecopy = pstrdup(value);
+		free(value);
+
+		(void) set_config_option(namecopy, valuecopy,
 								 context, source,
 								 action, true, 0, false);
 
-		free(name);
-		if (value)
-			free(value);
+		pfree(namecopy);
+		pfree(valuecopy);
 		pfree(s);
 	}
 }
@@ -10232,10 +10239,19 @@ static bool
 call_string_check_hook(struct config_string *conf, char **newval, void **extra,
 					   GucSource source, int elevel)
 {
+	volatile bool result = true;
+
 	/* Quick success if no hook */
 	if (!conf->check_hook)
 		return true;
 
+	/*
+	 * If elevel is ERROR, or if the check_hook itself throws an elog
+	 * (undesirable, but not always avoidable), make sure we don't leak the
+	 * already-malloc'd newval string.
+	 */
+	PG_TRY();
+	{
 		/* Reset variables that might be set by hook */
 		GUC_check_errcode_value = ERRCODE_INVALID_PARAMETER_VALUE;
 		GUC_check_errmsg_string = NULL;
@@ -10256,10 +10272,17 @@ call_string_check_hook(struct config_string *conf, char **newval, void **extra,
 					 errhint("%s", GUC_check_errhint_string) : 0));
 			/* Flush any strings created in ErrorContext */
 			FlushErrorState();
-		return false;
+			result = false;
 		}
+	}
+	PG_CATCH();
+	{
+		free(*newval);
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
 
-	return true;
+	return result;
 }
 
 static bool