layout.c: Cast argument of isspace() to unsigned char

These functions [isspace, etc] check whether c [(the argument)], which
must have the value of an unsigned char or EOF, falls into a certain
character class according to the current locale.

Cast the argument from `char` to `unsigned char` to guarantee that
we don't pass illegal negative values. Some implementations actually
provide a warning to get heads up[1].

[1] https://man.netbsd.org/ctype.3#CAVEATS

TEST=Builds on Cygwin (Windows 10, amd64, gcc 11.3.0)

Change-Id: Ia48d5a19b0964bc28e5360edf06bdf287dad5945
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/66548
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
---
M layout.c
1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/layout.c b/layout.c
index 39f504a..5916d66 100644
--- a/layout.c
+++ b/layout.c
@@ -312,7 +312,7 @@
 			entry->file = strdup(entry->name);
 
 		for (i = 0; entry->file[i]; ++i) {
-			if (isspace(entry->file[i]))
+			if (isspace((unsigned char)entry->file[i]))
 				entry->file[i] = '_';
 		}
 	}

