[musl] undefined behavior in getdelim.c

[Tamir Duberstein <tamird@google.com>]

Fuchsia's libc is derived from musl. We make extensive use of clang
sanitizers in Fuchsia, and UBSAN has found "applying zero offset to
null pointer" in getdelim.c.

Any call to `fopen` followed by a call to `getdelim` will trigger this
behavior. The UB happens at
https://git.musl-libc.org/cgit/musl/tree/src/stdio/getdelim.c#n59.
Immediately after `fopen` `f->rpos` is `NULL`; `rpos` won't be
initialized until a few lines down in `getcunlocked`.

Here's the stack trace from UBSAN in Fuchsia:
../../zircon/third_party/ulib/musl/src/stdio/getdelim.c:48:13: runtime
error: applying zero offset to null pointer
   #0    0x0000432ff5bf0613 in getdelim(char** restrict, size_t*
restrict, int, FILE* restrict)
../../zircon/third_party/ulib/musl/src/stdio/getdelim.c:48
<libc.so>+0x165613
   #1.2  0x00002380af30fe37 in ubsan_GetStackTrace()
compiler-rt/lib/ubsan/ubsan_diag.cpp:55 <libclang_rt.asan.so>+0x3be37
   #1.1  0x00002380af30fe37 in MaybePrintStackTrace()
compiler-rt/lib/ubsan/ubsan_diag.cpp:53 <libclang_rt.asan.so>+0x3be37
   #1    0x00002380af30fe37 in ~ScopedReport()
compiler-rt/lib/ubsan/ubsan_diag.cpp:389 <libclang_rt.asan.so>+0x3be37
   #2    0x00002380af3141fb in handlePointerOverflowImpl()
compiler-rt/lib/ubsan/ubsan_handlers.cpp:809
<libclang_rt.asan.so>+0x401fb
   #3    0x00002380af313d6d in
compiler-rt/lib/ubsan/ubsan_handlers.cpp:815
<libclang_rt.asan.so>+0x3fd6d
   #4    0x0000432ff5bf0613 in getdelim(char** restrict, size_t*
restrict, int, FILE* restrict)
../../zircon/third_party/ulib/musl/src/stdio/getdelim.c:48
<libc.so>+0x165613

Note that Fuchsia is a years behind, but I've confirmed this UB
happens even with the latest musl sources.

Fixing this should be quite straightforward. I'm happy to send a patch
if you agree.

Please CC me on response as I am not a subscriber to this mailing list
per the guidance on https://musl.libc.org/support.html.

Thank you.
Tamir