ld: do not emit reference to dynamic library named ""

Fixes #1778.

R=ken2
CC=golang-dev
https://golang.org/cl/4550047

src/cmd/5l/asm.c

@@ -89,6 +89,9 @@ needlib(char *name)
 	char *p;
 	Sym *s;
 
+	if(*name == '\0')
+		return 0;
+
 	/* reuse hash code in symbol table */
 	p = smprint(".dynlib.%s", name);
 	s = lookup(p, 0);

src/cmd/6l/asm.c

@@ -108,6 +108,9 @@ needlib(char *name)
 	char *p;
 	Sym *s;
 
+	if(*name == '\0')
+		return 0;
+
 	/* reuse hash code in symbol table */
 	p = smprint(".elfload.%s", name);
 	s = lookup(p, 0);

src/cmd/8l/asm.c

@@ -104,6 +104,9 @@ needlib(char *name)
 	char *p;
 	Sym *s;
 
+	if(*name == '\0')
+		return 0;
+
 	/* reuse hash code in symbol table */
 	p = smprint(".dynlib.%s", name);
 	s = lookup(p, 0);