Fix popen_test in MODE=dbg

ASAN and vfork() don't appear to play well together. Maybe in later
versions of GCC it'll be better. But vfork() is flirting with danger
after all and that probably doesn't make sense in ASAN mode anyway.

libc/stdio/fgetc.c

@@ -23,11 +23,11 @@
  * @return byte in range 0..255, or -1 w/ errno
  */
 int fgetc(FILE *f) {
-  unsigned char b;
+  unsigned char b[1];
   if (f->beg < f->end) {
     return f->buf[f->beg++] & 0xff;
   } else {
-    if (!fread(&b, 1, 1, f)) return -1;
-    return b;
+    if (!fread(b, 1, 1, f)) return -1;
+    return b[0];
   }
 }