Prevent Q_IsColorString from asserting on negative ascii chars

code/qcommon/q_shared.c

@@ -23,6 +23,28 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 // q_shared.c -- stateless support routines that are included in each code dll
 #include "q_shared.h"
 
+// ^[0-9a-zA-Z]
+qboolean Q_IsColorString(const char *p) {
+	if (!p)
+		return qfalse;
+
+	if (p[0] != Q_COLOR_ESCAPE)
+		return qfalse;
+
+	if (p[1] == 0)
+		return qfalse;
+
+	// isalnum expects a signed integer in the range -1 (EOF) to 255, or it might assert on undefined behaviour
+	// a dereferenced char pointer has the range -128 to 127, so we just need to rangecheck the negative part
+	if (p[1] < 0)
+		return qfalse;
+
+	if (isalnum(p[1]) == 0)
+		return qfalse;
+
+	return qtrue;
+}
+
 float Com_Clamp( float min, float max, float value ) {
 	if ( value < min ) {
 		return min;

code/qcommon/q_shared.h

@@ -409,7 +409,7 @@ extern	vec4_t		colorMdGrey;
 extern	vec4_t		colorDkGrey;
 
 #define Q_COLOR_ESCAPE	'^'
-#define Q_IsColorString(p)	((p) && *(p) == Q_COLOR_ESCAPE && *((p)+1) && isalnum(*((p)+1))) // ^[0-9a-zA-Z]
+qboolean Q_IsColorString(const char *p);  // ^[0-9a-zA-Z]
 
 #define COLOR_BLACK	'0'
 #define COLOR_RED	'1'