PAM: Do not skip multiple whitespaces after `ENDHDR`
PAM: Do not skip multiple whitespaces after `ENDHDR`

The parser was confusing a whitespace character with the actual first
pixel value.

Fixes the following test cases:

```
% convert -depth 8 -size 512x512 xc:#090909 tab.pam
% convert -depth 8 -size 512x512 xc:#202020 space.pam
% convert -depth 8 -size 512x512 xc:#0A0A0A lf.pam
% convert -depth 8 -size 512x512 xc:#0D0D0D cr.pam
```

```
% for i in tab.pam space.pam lf.pam cr.pam; do cjxl $i $i.jxl; done
Failed to read image tab.pam.
Failed to read image space.pam.
Failed to read image lf.pam.
Failed to read image cr.pam.
```

libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl