input: consume the whole escape sequence for modified PgUp and PgDn keys

Even when we don't act on most of these combinations, at least eat up
the whole sequence so the unknown keystroke doesn't enter things like
"3~" into the buffer.

This fixes https://savannah.gnu.org/bugs/?54291.

src/winio.c

@@ -1094,10 +1094,10 @@ int convert_sequence(const int *seq, size_t length, int *consumed)
 						if (length > 2 && (seq[2] == '~' || seq[2] == '^'))
 							return KEY_PPAGE;
 #ifndef NANO_TINY
-						else if (length > 4 && seq[2] == ';' &&
-									seq[3] == '2' && seq[4] == '~') {
+						else if (length > 4 && seq[2] == ';' && seq[4] == '~') {
 							*consumed = 5;
-							return shiftaltup;
+							if (seq[3] == '2')
+								return shiftaltup;
 						}
 #endif
 						break;
@@ -1107,10 +1107,10 @@ int convert_sequence(const int *seq, size_t length, int *consumed)
 						if (length > 2 && (seq[2] == '~' || seq[2] == '^'))
 							return KEY_NPAGE;
 #ifndef NANO_TINY
-						else if (length > 4 && seq[2] == ';' &&
-									seq[3] == '2' && seq[4] == '~') {
+						else if (length > 4 && seq[2] == ';' && seq[4] == '~') {
 							*consumed = 5;
-							return shiftaltdown;
+							if (seq[3] == '2')
+								return shiftaltdown;
 						}
 #endif
 						break;