glsl: fix crash on glsl macro redefinition

glsl: fix crash on glsl macro redefinition

In case shader contains two equal macro defines, first one with trailing spaces
and the second one without.
`#define A 1   `
`#define A 1`
The parser crashes

Fixes: 0346ad37741b11d640c1c4970b275c1f0c7f9e75 ("glsl: ignore trailing whitespace when define redefined")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5312>