Source: Avoid out-of-range inputs to std::isspace()

`isspace` takes `int` but documents that the value must be representable
by `unsigned char`, or be EOF.  Use a wrapper to cast to `unsigned char`
to avoid sign extension while converting to `int`.  This generalizes the
fix from commit 5e8c176e2a (cmExecuteProcessCommand: Cast c to unsigned
char before cast to int, 2024-01-05) to other `isspace` call sites.

This was detected by assertions in the MSVC standard library while
processing UTF-8 text.

Issue: #25561

Source/cmStringCommand.cxx

@@ -6,7 +6,6 @@
 #include "cmStringCommand.h"
 
 #include <algorithm>
-#include <cctype>
 #include <cstdio>
 #include <cstdlib>
 #include <limits>
@@ -660,7 +659,7 @@ bool HandleStripCommand(std::vector<std::string> const& args,
   const char* ptr = stringValue.c_str();
   size_t cc;
   for (cc = 0; cc < inStringLength; ++cc) {
-    if (!isspace(*ptr)) {
+    if (!cmIsSpace(*ptr)) {
       if (startPos > inStringLength) {
         startPos = cc;
       }