ld: fix segfault when linker script is not found

ld: fix segfault when linker script is not found

ld previously crashed with a segmentation fault if the specified linker
script could not be found. The issue seems to have been introduced
recently by d048eee2910 [1].

This patch adds a check to ensure that a filename was found after
searching the possible prefixes. If no filename was found, the function
returns NULL, and ldfile_open_command_file_1() emits a proper fatal
error message.
This change prevents the crash and provides a clear diagnostic.

A new generic test was also added to cover this error case.

[1]: https://inbox.sourceware.org/binutils/20250812143757.3565482-1-hjl
     .tools@gmail.com/