XUtils: add NULL pointer guard in String_contains_i - #2152

Summary

Fixes: #1864

In issue #1864, a segmentation fault (Signal 11) occurred on macOS during process table filtering:

strcasestr_l + 132
String_contains_i + 156
Process_rowMatchesFilter + 108
Table_rebuildPanel + 372
ScreenManager_run + ...

While the exact user action that activated the filter in the reported incident is unknown (e.g. interactive F4 filter entry vs. -F command-line flag), the backtrace establishes that an incremental filter was active when the crash occurred (Process_rowMatchesFilter only invokes String_contains_i when table->incFilter is non-NULL).

On Darwin/macOS (such as unnamed threads or unpopulated comm entries) and Linux (kernel threads and zombies), Process_getCommand() can legitimately return NULL. String_contains_i() previously passed s1 directly to strcasestr(), causing an immediate crash when encountering a NULL command.

Fix

Add an early guard at the beginning of String_contains_i():

if (!s1 || !s2)
   return false;

If either the haystack or needle is NULL, the function safely returns false. In Process_matchesFilter(), !String_contains_i(...) evaluates to true, correctly filtering out the entry without crashing.

Verification

• Verified against the compiled XUtils.o across 9 regression test cases in an external test harness covering NULL haystacks, NULL needles, both pointers NULL, single/multi-needle matching, and multi-needle NULL permutations (unpatched failed 5/9 cases with SIGSEGV; patched passed 9/9).
• Verified full call path against compiled Process.o + XUtils.o with mock unnamed thread entries (proc.cmdline = NULL), confirming graceful filtering without crashing.
• Verified git diff --check, ./check-pcp-style.sh, and make check pass with zero warnings or errors.