runtime: fix handling of GOTRACEBACK

Since CL 130990043, the GOTRACEBACK variable is
only used when the GODEBUG variable is set.
This change restores the original behavior.

LGTM=rsc
R=golang-codereviews, aram, gobot, r, rsc
CC=golang-codereviews
https://golang.org/cl/132520043

src/runtime/runtime.c

@@ -287,8 +287,7 @@ runtime·parsedebugvars(void)
 	intgo i, n;
 
 	p = runtime·getenv("GODEBUG");
-	if(p == nil)
-		return;
+	if(p != nil){
 		for(;;) {
 			for(i=0; i<nelem(dbgvar); i++) {
 				n = runtime·findnull((byte*)dbgvar[i].name);
@@ -300,6 +299,7 @@ runtime·parsedebugvars(void)
 				break;
 			p++;
 		}
+	}
 
 	p = runtime·getenv("GOTRACEBACK");
 	if(p == nil)