html.c: avoid out-of-bounds access for url_escape_table

This fixes a segfault for me with with -O2 optimization on x86
with gcc (Debian 4.4.5-8) 4.4.5

I can reliably reproduce it with the following parameters
when pointed to the git.git repository:

PATH_INFO='/git-core.git/diff/'
QUERY_STRING='id=2b93bfac0f5bcabbf60f174f4e7bfa9e318e64d5&id2=d6da71a9d16b8cf27f9d8f90692d3625c849cbc8'

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

html.c

@@ -162,7 +162,7 @@ void html_url_path(const char *txt)
 {
 	const char *t = txt;
 	while(t && *t){
-		int c = *t;
+		unsigned char c = *t;
 		const char *e = url_escape_table[c];
 		if (e && c!='+' && c!='&') {
 			html_raw(txt, t - txt);
@@ -179,7 +179,7 @@ void html_url_arg(const char *txt)
 {
 	const char *t = txt;
 	while(t && *t){
-		int c = *t;
+		unsigned char c = *t;
 		const char *e = url_escape_table[c];
 		if (c == ' ')
 			e = "+";