Fix error reported by ASAN in 590.asan.0.7288.pdf

==31898== ERROR: AddressSanitizer: heap-use-after-free on address 0x60ae0007432c at pc 0x7f03483026aa bp 0x7fff6ec0c820 sp 0x7fff6ec0c818
READ of size 4 at 0x60ae0007432c thread T0
    #0 0x7f03483026a9 in SplashFTFont::getGlyphPath(int) /home/tsdgeos/devel/poppler/splash/SplashFTFont.cc:414
    #1 0x7f034829f681 in SplashOutputDev::drawChar(GfxState*, double, double, double, double, double, double, unsigned int, int, unsigned int*, int) /home/tsdgeos/devel/poppler/poppler/SplashOutputDev.cc:2239
    #2 0x7f0348100599 in Gfx::doShowText(GooString*) /home/tsdgeos/devel/poppler/poppler/Gfx.cc:4054
    #3 0x7f03480fddb0 in Gfx::opShowSpaceText(Object*, int) /home/tsdgeos/devel/poppler/poppler/Gfx.cc:3886
    #4 0x7f03480dc33a in Gfx::execOp(Object*, Object*, int) /home/tsdgeos/devel/poppler/poppler/Gfx.cc:903
    #5 0x7f03480db50f in Gfx::go(bool) /home/tsdgeos/devel/poppler/poppler/Gfx.cc:762
    #6 0x7f03480db163 in Gfx::display(Object*, bool) /home/tsdgeos/devel/poppler/poppler/Gfx.cc:728
    #7 0x7f03481bddd1 in Page::displaySlice(OutputDev*, double, double, int, bool, bool, int, int, int, int, bool, bool (*)(void*), void*, bool (*)(Annot*, void*), void*, bool) /home/tsdgeos/devel/poppler/poppler/Page.cc:585
    #8 0x7f03481c52fd in PDFDoc::displayPageSlice(OutputDev*, int, double, double, int, bool, bool, bool, int, int, int, int, bool (*)(void*), void*, bool (*)(Annot*, void*), void*, bool) /home/tsdgeos/devel/poppler/poppler/PDFDoc.cc:503
    #9 0x40311e in savePageSlice(PDFDoc*, SplashOutputDev*, int, int, int, int, int, double, double, char*) /home/tsdgeos/devel/poppler/utils/pdftoppm.cc:222
    #10 0x404416 in main /home/tsdgeos/devel/poppler/utils/pdftoppm.cc:521
    #11 0x7f03477e5ec4 (/lib/x86_64-linux-gnu/libc.so.6+0x21ec4)
    #12 0x401d58 in _start (/home/tsdgeos/devel/poppler/build-debug/utils/pdftoppm+0x401d58)

splash/SplashFTFont.cc

@@ -410,7 +410,7 @@ SplashPath *SplashFTFont::getGlyphPath(int c) {
   ff->face->size = sizeObj;
   FT_Set_Transform(ff->face, &textMatrix, NULL);
   slot = ff->face->glyph;
-  if (ff->codeToGID && c < ff->codeToGIDLen) {
+  if (ff->codeToGID && c < ff->codeToGIDLen && c >= 0) {
     gid = ff->codeToGID[c];
   } else {
     gid = (FT_UInt)c;