hush: optimize ${var/pattern/repl} for trivial patterns

function                                             old     new   delta
expand_one_var                                      2353    2507    +154

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/match.c b/shell/match.c
index ee8abb2..90f7754 100644
--- a/shell/match.c
+++ b/shell/match.c
@@ -64,11 +64,10 @@
 	}
 
 	while (loc != end) {
-		char c;
 		int r;
 
-		c = *loc;
 		if (flags & SCAN_MATCH_LEFT_HALF) {
+			char c = *loc;
 			*loc = '\0';
 			r = fnmatch(pattern, string, 0);
 			//bb_error_msg("fnmatch('%s','%s',0):%d", pattern, string, r);