Fix compilation warning in jemalloc's malloc_vsnprintf (#7789)

Change `val` to `unsigned char` before being tested.
The fix is identical to the one that's been made in upstream jemalloc.

warning is:
src/malloc_io.c: In function ‘malloc_vsnprintf’:
src/malloc_io.c:369:2: warning: case label value exceeds maximum value for type
  369 |  case '?' | 0x80:      \
      |  ^~~~
src/malloc_io.c:581:5: note: in expansion of macro ‘GET_ARG_NUMERIC’
  581 |     GET_ARG_NUMERIC(val, 'p');
      |     ^~~~~~~~~~~~~~~

deps/jemalloc/src/malloc_io.c

@@ -362,7 +362,7 @@ malloc_vsnprintf(char *str, size_t size, const char *format, va_list ap) {
 	}								\
 } while (0)
 #define GET_ARG_NUMERIC(val, len) do {					\
-	switch (len) {							\
+	switch ((unsigned char)len) {					\
 	case '?':							\
 		val = va_arg(ap, int);					\
 		break;							\