addStrokeAdjustHint(): fix crash in out-of-memory situation.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=25411

```
    #0 0xf7ef8f19 in [vdso]
    #1 0xf7ccdd08 in gsignal (/lib32/libc.so.6+0x2bd08)
    #2 0xf7ccf206 in abort (/lib32/libc.so.6+0x2d206)
    #3 0xbdb9c2e in grealloc(void*, unsigned int, bool) gdal/poppler/goo/gmem.h:85:5
    #4 0xbdd9e11 in greallocn(void*, int, int, bool, bool) gdal/poppler/goo/gmem.h:171:12
    #5 0xc012373 in SplashPath::addStrokeAdjustHint(int, int, int, int) gdal/poppler/splash/SplashPath.cc:211:35
    #6 0xbfd156f in Splash::makeStrokePath(SplashPath*, double, bool) gdal/poppler/splash/Splash.cc:5987:34
    #7 0xbfcaec2 in Splash::strokeWide(SplashPath*, double) gdal/poppler/splash/Splash.cc:2028:13
    #8 0xbfc8a4d in Splash::stroke(SplashPath*) /src/gdal/poppler/splash/Splash.cc
```
7 jobs for fix_crash_in_addStrokeAdjustHint in 4 minutes and 55 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #4416962
build

00:04:55

passed #4416967
build_android

00:00:52

passed #4416963
build_clang_libcpp

00:04:08

passed #4416966
build_clazy

00:02:07

passed #4416965
build_mingw64_fedora

00:03:14

passed #4416964
build_ubuntu_16_04

00:02:40

passed #4416961
clang_format

00:01:05