Never pass a signed char to ctype macros like isdigit()

Bug: None
Change-Id: I451bb2c1f175a77aefbc8363009bf35a769fe941
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/264442
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37037}
diff --git a/modules/audio_coding/test/PCMFile.cc b/modules/audio_coding/test/PCMFile.cc
index 659f968..9ccfa49 100644
--- a/modules/audio_coding/test/PCMFile.cc
+++ b/modules/audio_coding/test/PCMFile.cc
@@ -62,7 +62,9 @@
   int16_t n = 0;
 
   // Removing trailing spaces.
-  while ((isspace(tmp_name[n]) || iscntrl(tmp_name[n])) && (tmp_name[n] != 0) &&
+  while ((isspace(static_cast<unsigned char>(tmp_name[n])) ||
+          iscntrl(static_cast<unsigned char>(tmp_name[n]))) &&
+         (static_cast<unsigned char>(tmp_name[n]) != 0) &&
          (n < MAX_FILE_NAME_LENGTH_BYTE)) {
     n++;
   }
@@ -73,7 +75,9 @@
   // Removing trailing spaces.
   n = (int16_t)(strlen(tmp_name) - 1);
   if (n >= 0) {
-    while ((isspace(tmp_name[n]) || iscntrl(tmp_name[n])) && (n >= 0)) {
+    while ((isspace(static_cast<unsigned char>(tmp_name[n])) ||
+            iscntrl(static_cast<unsigned char>(tmp_name[n]))) &&
+           (n >= 0)) {
       n--;
     }
   }