runtime: avoid crash in Caller

Fixes #176.

R=r
https://golang.org/cl/154146

src/pkg/runtime/386/traceback.c

@@ -42,7 +42,7 @@ traceback(byte *pc0, byte *sp, G *g)
 		if(f == nil) {
 			// dangerous, but poke around to see if it is a closure
 			// ADDL $xxx, SP; RET
-			if(p != 0 && p[0] == 0x81 && p[1] == 0xc4 && p[6] == 0xc3) {
+			if((uint64)pc > 0x1000 && p[0] == 0x81 && p[1] == 0xc4 && p[6] == 0xc3) {
 				sp += *(uint32*)(p+2) + 8;
 				pc = *(uintptr*)(sp - 8);
 				if(pc <= 0x1000)

src/pkg/runtime/amd64/traceback.c

@@ -127,7 +127,7 @@ runtime·Caller(int32 n, uint64 retpc, String retfile, int32 retline, bool retbo
 			// see if it is a closure.
 			p = (byte*)pc;
 			// ADDQ $xxx, SP; RET
-			if(p[0] == 0x48 && p[1] == 0x81 && p[2] == 0xc4 && p[7] == 0xc3) {
+			if(pc > 0x1000 && p[0] == 0x48 && p[1] == 0x81 && p[2] == 0xc4 && p[7] == 0xc3) {
 				sp += *(uint32*)(p+3) + 8;
 				goto loop;
 			}