fix: Fix parseExp reading oob during parseFloat (#1567)

Author: MaxGraey

std/assembly/util/string.ts

@@ -1123,6 +1123,7 @@ function parseExp(ptr: usize, len: i32): i32 {
   // check code is 'e' or 'E'
   if ((code | 32) != CharCode.e) return 0;
 
+  if (!--len) return 0;
   code = <u32>load<u16>(ptr += 2);
   if (code == CharCode.MINUS) {
     if (!--len) return 0;