bin/48856: make dumps core on invalid assignment

[Thomas Klausner <wiz%NetBSD.org@localhost>]

        Note: There was a bad value `' for the field `Class'.
        It was set to the default value of `sw-bug'.

>Number:         48856
>Category:       bin
>Synopsis:       make dumps core on invalid assignment
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri May 30 22:45:00 +0000 2014
>Originator:     Thomas Klausner
>Release:        NetBSD 6.99.43
>Organization:
Curiosity is the very basis of education and if you tell me that 
curiosity killed the cat, I say only that the cat died nobly.
- Arnold Edinborough
>Environment:
        
        
Architecture: x86_64
Machine: amd64
>Description:
With the following Makefile, make dumps core:

--- begin ---
{prefix}=/usr/local
--- end ---

I arrived at this by accident, and I don't expect it to work,
but I don't expect make to dump core either.
>How-To-Repeat:
See above.

The backtrace is:
#0  0x000000000040feae in Parse_DoVar (line=line@entry=0x7f7ff7ff1000 
"{prefix}=/usr/local", ctxt=0x7f7ff7b01180) at 
/archive/foreign/src/usr.bin/make/parse.c:1813
1813        for (depth = 0, cp = line + 1; depth != 0 || *cp != '='; cp++) {
(gdb) bt
#0  0x000000000040feae in Parse_DoVar (line=line@entry=0x7f7ff7ff1000 
"{prefix}=/usr/local", ctxt=0x7f7ff7b01180) at 
/archive/foreign/src/usr.bin/make/parse.c:1813
#1  0x000000000041081b in Parse_File (name=name@entry=0x7f7ff7b222c9 
"Makefile", fd=<optimized out>) at 
/archive/foreign/src/usr.bin/make/parse.c:2993
#2  0x000000000040b8aa in ReadMakefile (p=<optimized out>, q=<optimized out>) 
at /archive/foreign/src/usr.bin/make/main.c:1460
#3  0x000000000041b3ac in Lst_FindFrom (l=0x7f7ff7ff1000, ln=0x7f7ff7b222e0, 
d=0x0, cProc=0x40b7dc <ReadMakefile>) at 
/archive/foreign/src/usr.bin/make/lst.lib/lstFindFrom.c:83
#4  0x000000000041b36b in Lst_Find (l=<optimized out>, d=d@entry=0x0, 
cProc=cProc@entry=0x40b7dc <ReadMakefile>) at 
/archive/foreign/src/usr.bin/make/lst.lib/lstFind.c:72
#5  0x000000000041c277 in main (argc=1, argv=<optimized out>) at 
/archive/foreign/src/usr.bin/make/main.c:1220

>Fix:
Not provided.

>Unformatted: